Racks Python API and _get in base.Manager

This adds _get function to base.Manager and basic read actions for Racks.

If you want to do manual testing, you'll have to fire up Python REPL:
```python
from tuskarclient.common import http
from tuskarclient.v1_0 import racks
api = http.HTTPClient('http://f18-tuskar:6385')
rm = racks.RackManager(api)

# and now you can play
rm.list()
rm.get(1) # provided that rack with id 1 exists
```