Update CONTRIBUTING to point to Contributing guide

The guide has a lot more info, and mentions Rainforest.
[Gemspec] Add dependency on cocoapods-trunk
[Gemfile] Update for renamed cocoapods-trunk repo
[Specification] Remove Ruby hooks

https://github.com/CocoaPods/CocoaPods/issues/2151\#issuecomment-43493702
[Command::Push] Disable pushing to the master repo

https://github.com/CocoaPods/CocoaPods/issues/2151\#issuecomment-43493702\
[Command::Push] Move to Command::Repo
[Integration] Fix
[Command::Push] Add temporary foward command
[Command::Push] Add temporary foward command 2
Merge pull request #2153 from CocoaPods/trunk

Trunk