Basic support for external issues tracker

Implementation of feature discussed  there https://github.com/gitlabhq/gitlabhq/issues/97#issuecomment-9193798

Looks like this:

![Screenshot-Admin area GitLab - Google Chrome](https://f.cloud.github.com/assets/672631/148645/a8fc7b20-750b-11e2-9120-f245ecc3d4c8.png)