Feature: Python Library allows for Plain File Retrieval

Now you can do something like:

```python
import cvmfs

repo        = cvmfs.RemoteRepository("http://cvmfs-stratum-one.cern.ch/opt/atlas")
clg         = repo.retrieve_root_catalog()
dirtab      = clg.find_directory_entry("/.cvmfsdirtab")
dirtab_file = dirtab.retrieve_from(repo)
print dirtab_file.read()
```

At the moment I find that code quite verbose. I'll leave the convenience API as a TODO for now.