Added a callback feature to all tasks

Hi,

I replicated [this pull request](https://github.com/rubenv/grunt-git/pull/88) after I checked out all jscs formatting issues. 

A i mentioned in the previous PR, this edit came out after I needed to move files into another folder after they have been downloaded with a gitpull task.

Modified code chunks lay at line 21 and lines from 27 to 41.

Please consider I can missing something as I'm not an expert grunt-plugin developer. Feel free to ask or modify given code.

Thanks

Emanuele