Feature: Simple Catalog Manager

This factors out the catalog loading functionality from the `WritableCatalogManager` into an intermediate class called `SimpleCatalogManager`. It can be used f.e. by `cvmfs_swissknife` utilities to get full access to a catalog manager but without the writing functionality.

**Note:** This contains the Pull Requests [Feature: Pathspec classes](https://github.com/cvmfs/cvmfs/pull/448) and [Feature: Dirtab Wrapper Class](https://github.com/cvmfs/cvmfs/pull/449).