Add nullability annotations to public API

[nolanw]

As mentioned in #105.

Nullability annotations were introduced in Xcode 6.3 (which included iOS 8.3). I'm not sure what the minimum supported Xcode version is in these parts, but if 6.3 is a reasonable minimum then we shouldn't need any #define NS_ASSUME_NONNULL_BEGIN etc. to pacify older versions of the dev tools. Let me know if I've guessed wrong here.

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

:memo: Please visit https://cla.developers.google.com/ to sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.
• If your company signed a CLA, they designated a Point of Contact who decides which employees are authorized to participate. You may need to contact the Point of Contact for your company and ask to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the project maintainer to go/cla#troubleshoot. The email used to register you as an authorized contributor must be the email used for the Git commit.
• In order to pass this check, please resolve this problem and have the pull request author add another comment and the bot will run again. If the bot doesn't comment, it means it doesn't think anything has changed.

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

:memo: Please visit https://cla.developers.google.com/ to sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.
• If your company signed a CLA, they designated a Point of Contact who decides which employees are authorized to participate. You may need to contact the Point of Contact for your company and ask to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the project maintainer to go/cla#troubleshoot. The email used to register you as an authorized contributor must be the email used for the Git commit.
• In order to pass this check, please resolve this problem and have the pull request author add another comment and the bot will run again. If the bot doesn't comment, it means it doesn't think anything has changed.

[nolanw]

I signed it!

[googlebot]

CLAs look good, thanks!

[morganchen12]

@mcdonamp please merge when you get a moment

[asciimike]

@nolanw, @morganchen12 Sorry about the delay. One minor change on the public surface; otherwise, LGTM. Thanks for doing this!

[nolanw]

Not nullable it is!

[asciimike]

LGTM

[asciimike]

@morganchen12, do you have permissions to release, or do you need me to?

[morganchen12]

I don't have release permissions, so please make a new release when you have the time.

Thanks!

[asciimike]

Published in v2.0.1.