Added a callback feature to all tasks

Added a callback feature to all tasks by modifying few lines in ` /tasks/git.js`  file.

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 (some lines are marked as modified just because we use different code-beautifier tool).


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