Feature: Pathspec classes

These are wrapper classes to process a `.cvmfsdirtab`. The main class is `Pathspec`, which is able to parse a path rules as seen in `.cvmfsdirtab`. Currently it supports:
* Parsing of shell-like path-patterns
* Matching a given path string against the pattern
* Decompose the path-pattern into a `GlobSequence`, i.e. a single glob pattern per directory-level
* Comparison of individual `Pathspecs`

Additionally this contains a unit test suite for the `Pathspec` classes.