Feature: BashCcompletion for cvmfs_server tag

This adds basic bash completion for `cvmfs_server tag`. As a cherry on top, one can do:

```bash
cvmfs_server tag -a [-r] [-i] <TAB><TAB>
generic-2014-11-13T12:54:01Z  generic-2014-11-13T13:06:41Z  trunk 
```