Adding git capacities to eXide

### This pull request adds some basic git capacities to eXide

It does nothing really fancy, but allows to: 
- perform a ```git commit``` after an application synchronization
- view available git branch and perform a ```git checkout``` to a new branch 

To enable the new git-capacities for a specific application, you must include a ```git.xml``` file in the application folder :
```xml
<git xmlns="http://exist-db.org/eXide/git">
    <workingDir>/home/yourUserName/...</workingDir>
</git>
```
With workingDir pointing to your git version control directory. This will enable a new Git menu and display the current git branch on eXide.

Cheers,
C. 

![screenshot from 2013-09-12 12 52 57](https://f.cloud.github.com/assets/1168053/1129986/f8421aa2-1b99-11e3-95df-770baf4b8ff4.png)