version bump + updating changelog
Merge branch 'release/0.5.0'
Merge tag 'v0.5.0' into dev

v0.5.0 release
 * Changing `User#favorite()` to use `PUT` instead of `POST`
 * Adds `Organization#participation()` (taken directly from `User`)
 * Changing `User#participation()` to have a 3rd param `key` (changes signature, not backwards compatible)