Getter/Setter && Page Actions

[DrewML]

This PR has 2 primary updates

1. It switches the extension to using a getter/setter to capture when angular is loaded, and hook bootstrapping. This is in contrast to our previous method, which listened for scripts being appended to the DOM and for DOMContentLoaded. While the old method created a race-case for how quickly we could patch bootstrapping, this new method ensures that we always wrap the bootstrap method during it's assignment.
2. The toggle button in Chrome has been switched to use a pageAction. It will now only show the toggle to a user (in the address bar of a tab) when Angular is present on the page.

[DrewML]

Changing the way we bootstrap actually ended up fixing 1 or 2 bugs I wasn't aware existed. Check it out:

It looks like this PR somehow addressed issues with

1. Scope "creators" not showing
2. Multiple apps not being detected.

[DrewML]

Found at least one site that doesn't work with this (yet). Since this is such a big change to how bootstrapping currently works, I'm going to keep testing on a bunch of Angular sites and see if I can find any other issues.

Sites that worked prior to change, but don't work now:

• https://preadly.com/
• http://allrecipes.com/ (works, but missing some scope data) It turns out that this site is calling bootstrap twice with null (instead of an element). This didn't cause failures previously because the bootstrap calls were made before our script monkey-patched the bootstrap prop (which was fixed in this PR). I'll open a separate issue for us to fail soft when that happens.
• https://www.patreon.com/ (Breaks entire site. Sites code assigns window.angular twice, which causes us to try and set a setter on a setter for bootstrap. Need to add a check for this use-case, although I'd assume and hope it's rare)

[DrewML]

Note: This PR appears to break Batarang, when Batarang is set to "Enabled" in it's DevTools tab, and ng-inspector is installed.

I'll need to figure out a solution to that prior to us moving these changes in.