Show committer only when needed.

# Summary
It is based on https://github.com/travis-ci/travis-web/pull/267. I can rebase, once it will be merged.

* a little cleanup in `spec_helper`
* add `authorIsCommitter` method to `Commit` model
* a little logic in build show template

# Preview
![screenshot-by-nimbus 33](https://cloud.githubusercontent.com/assets/193936/2923551/09715748-d71a-11e3-98b3-829a00852b20.png)
![screenshot-by-nimbus 34](https://cloud.githubusercontent.com/assets/193936/2923556/1531dc74-d71a-11e3-9203-3175756c5b4a.png)