Add limited support for Google App Engine.

[dmitshur]

This allows the package to build and operate in a limited fashion on Google App Engine, where "unsafe" package is not available.

Closes #35.

This PR is simply for your convenience @davecgh, feel free to make use of it or close it depending on your plan to handle #35.

[davecgh]

Thanks for the PR.

I believe there are a couple more things that need to be considered here before it's ready however:

• I'm pretty sure the code will panic with this PR on GAE since I believe it makes assumptions that after a call to unsafeReflectValue the returned reflect.Value is addressable and therefore can have .Addr and .Interface on it. Since the stub won't actually make it addressable, those calls will panic. Thus, there needs to be some type of constant that is set differently when running in "unsafe" mode versus not and the code would need to be updated to respect that constant accordingly.
• The tests are going to fail on GAE since they rely on being able to make use of unsafeReflectValue. The output when that is a stub is going to be different which will result in failing tests. The tests will need to updated to handle both variants accordingly, likely by using the same constant mechanism previously mentioned.

[dmitshur]

Makes sense. It seems like you will need to take care of significantly more things to resolve that issue than this PR offers. I'll close it. As I said in the description, I made it just in case it's helpful, and it took me a minute to do it because I backported my go-goon/bypass change. I did not realize you'll have extra considerations, but that makes sense.

[davecgh]

np. Thanks for the PR though. It'll still be useful even if a few more things have to be considered.