Show pull request link, when commit related with pull request

![2014-10-27 4 19 06](https://cloud.githubusercontent.com/assets/683590/4785226/51eabd8c-5d77-11e4-8f86-8b0c8c7c2b4f.png)
![2014-10-27 4 19 18](https://cloud.githubusercontent.com/assets/683590/4785227/51f5437e-5d77-11e4-9d9d-e25a7e972e95.png)

We should use the appropriate scheme here (`http` vs `https`). We can use the `Repo.URL` field for this:
https://github.com/drone/drone/blob/master/shared/model/repo.go#L27

This field is populated for GitHub + GitHub enterprise. It should be set of other providers as well.