Add changelog for 2.0

An attempt to capture all the 2.0 changes in a better format, and include information to help consumers migrate. Once this is merged, the changes list in #618 can be replaced with a link to this file.

As an aside, I don't think minor and patch versions need to be as detailed as this.

Fixes #679.

[Rendered Markdown](https://github.com/ReactiveCocoa/ReactiveCocoa/blob/add-changelog/CHANGELOG.md)