Update documentation

* Move changelog into new file.
* Add release dates to change log.
* Change copyright dates to 2011, the first year of public release.
* Remove email addresses / domains for author tag to stop end-users who buy
plugins and themes that use TGMPA, from thinking that we're the folks to
contact to support them with those plugins and themes. Enough already!
Fix undefined index notice with $plugin['required']

Only shows up with WP_DEBUG enabled. Reported by @Michi23.

Fixes #215.