Add a "make repoinfo" command

This will add a command that creates a file containing information about the current repository version (the SHA).
The information is displayed in the debug toolbar by this commit: 8103fcb6952823ca627f5803ee27fe50885c750e

Currently it looks like this:
![debug-toolbar](https://f.cloud.github.com/assets/33978/1582541/4955d82a-51f0-11e3-8222-b539460a294b.png)
Example: `1.0.0-beta-58-g63e754e9a5-dirty`
Which is 
* The last tag [1.0.0-beta](https://github.com/joomla/jissues/releases/tag/1.0.0-beta)
* `58` commits ahead
* The "current SHA"  63e754e9a5
* The "state" is `dirty` (uncommitted changes)

**Note**: I assume that this will currently not work in Windows (`git` executable ..)