Python Module for easy Catalog Inspection + walkCatalogs.py

This contains some basic Python classes that allow for (interactive) CVMFS catalog inspection. This basically consolidates most of the scripts in `add-ons/tools` and makes the code re-usable. Furthermore `cvmfs.py` is designed to be a simple python module allowing for interactive catalog inspection of both remote and local catalog hierarchies.

**NOTE:**
These scripts are designed for debugging and *NOT* for performance!

**Example Session:**
```
$ python
>>> import cvmfs
>>> repo = cvmfs.OpenRepository("http://cvmfs-stratum-one.cern.ch/opt/lhcb/")
>>> repo.manifest.revision
14969
>>> root = repo.RetrieveRootCatalog()
>>> root.OpenInteractive()
sqlite> SELECT count(*) FROM catalog;
314023
sqlite> .q
>>> root.ListNested()
[<CatalogReference for /lib/lhcb/ALIGNMENT>, <CatalogReference for /lib/lhcb/ANALYSIS>, ....]
>>> nested = repo.RetrieveCatalogForPath("/lib/lhcb/ALIGNMENT")
>>> nested.schema
1.2
>>> print nested.last_modified
2013-09-06 16:10:20
>>> nested.RunSql("SELECT md5path_1, md5path_2, name FROM catalog WHERE size > 2600000;")
[(-6401023936411038044L, 3491366632274530332L, 'libAlignTrTools.so')]
>>> entry = nested.FindDirectoryEntry(-6401023936411038044L, 3491366632274530332L)
>>> entry
<DirectoryEntry for libAlignTrTools.so>
>>> entry.BacktracePath(nested, repo)
'/lib/lhcb/ALIGNMENT/ALIGNMENT_v9r0/Alignment/AlignTrTools/x86_64-slc5-gcc43-opt/libAlignTrTools.so'
>>> quit()
$
```