Show current branch at shellPrompt

This pull-req shows current branch at shellPrompt.
This feature will be activated with this setting in ~/.sbt/build.sbt

```scala
showCurrentGitBranch
```

![git_prompt](https://f.cloud.github.com/assets/264469/680679/653c5b70-d998-11e2-9c55-91c50e167566.png)