Extract all Issue Tracker in gems

There are a lot of issue tracker connection on errbit and it's good. But they add a lot of dependencies by default and can't be update in independance of Errbit.

This Pull Request add a plugin system to extract them in their own gems.

A new repository is create to be the top level dependencies on issue tracker in first. More in future.

https://github.com/errbit/errbit_plugin

I start the github plugin because is highly integrate on Errbit and can't be easily not be a dependencies.

https://github.com/errbit/errbit_github_plugin

The next plan is now

 - [] Add test on errbit_github_plugin
 - [] Create new gem with all issue trackers already in Errbit
 - [] Test it a lot to be sure it's works

All review are welcome