Add-On: Python Tools based on CVMFS Python Library

This deprecates the python debug tools in `add-ons/tools` and replaces them by some new scripts that are based on the CVMFS python library in `python/`. I will add more of those scripts opportunistically as soon as I need them. For now there are some statistics gathering tools:

* `get_info.py` - retrieve the internal catalog statistics for the current root catalog of a repository
* `get_root_hash.py` - find the current root catalog hash of a repository
* `get_referenced_hashes.py` - walk the full catalog tree of a repository and list all referenced hashes
* `get_head_statistics.h` - a combination of the scripts above 

**Note:** This is based on [Pull Request #557](https://github.com/cvmfs/cvmfs/pull/557).