Update CHANGELOG and pubspec for 2.2.0.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=140605397

Add to README.md:

- Add a pub badge
- Add a gitter badge
- Add links to the popular AdWords/AdSense posts about Angular

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=140387396