Fix: Negative Dirtab Entries With Wildcards

This adds the possibility to have negative `.cvmfsdirtab` entries that are non-absolute paths with wildcards. Something like:
```
! *.svn
```
This will prevent CernVM-FS to create any nested catalogs inside a `.svn` directory.
See Jira ticket [CVM-639](https://sft.its.cern.ch/jira/browse/CVM-639).

**Note:** This Pull Request contains [Pull Request #517](https://github.com/cvmfs/cvmfs/pull/517).