Feature: Python Library can Traverse Catalogs

I needed that for some statistics generation. The python tools now provide a convenient way to traverse all catalogs and do things on them like so:

``` python
import cvmfs
repo = cvmfs.LocalRepository("test.cern.ch")
for catalog in repo.catalogs():
    for res in catalog.run_sql("SELECT lower(hex(hash)) FROM catalog WHERE hash != 0"):
        print res[0]
```

This would print all SHA1 hashes the current catalog revision of **test.cern.ch** has references to.