Feature: cvmfs_swissknife dirtab

This integrates the new `.cvmfsdirtab` functionality into the `cvmfs_swissknife` utility. It can be invoked by `cvmfs_swissknife dirtab` and processes a `.cvmfsdirtab` file without opening all catalogs of a repository.

**Note:** This contains the Pull Requests [Feature: Pathspec classes](https://github.com/cvmfs/cvmfs/pull/448), [Feature: Dirtab Wrapper Class](https://github.com/cvmfs/cvmfs/pull/449) and [Feature: Simple Catalog Manager](https://github.com/cvmfs/cvmfs/pull/450).