Feature: Dirtab Wrapper Class

This contains a wrapper class for the `.cvmfsdirtab` file. The class is responsible for parsing and evaluation of the dirtab rules using the `Pathspec` classes.

Currently supported features are:
* Parsing of both a file and a provided string
* Negative and Positive `Pathspec` rules
* Comments in `.cvmfsdirtab` (by using a `#` as first character in the line)
* Checking individual paths for conformance with the dirtab

Additionally this contains a unit test suite for the dirtab implementation.

**Note:** This contains the Pull Request [Feature: Pathspec classes](https://github.com/cvmfs/cvmfs/pull/448)