Bumped Firebase dependency to 2.0.0

@mimming - Please review and merge. I will do a ReactFire 0.4.0 release once this gets merged in. Here are the changes:
* Bumped Firebase dependency to 2.0.0.
* Bumped React to [0.12.0, their latest version](https://facebook.github.io/react/blog/2014/10/28/react-v0.12.html).
* Removed `envify` as an npm dependency. [React fixed this issue](https://github.com/facebook/react/commit/486b3c0bc5fc104d3b106b4ab58136b392753150) which required us to include `envify` as a peer dependency of React, so we no longer need to include it.
* Updated test suite to not use any deprecated Firebase methods.
* Updated examples to not use any deprecated Firebase methods.
* Updated changelog for upcoming 0.4.0 release.