Integration of Escalator from playground
use_list=False for msgpack.unpackb
PEP8
Use __all__ in __init__.py files
Install LevelDB on Travis
Use a ThreadDevice and inproc for the workers
Fix LevelDB installation on Travis
Use command-line argument to bind server address
Fix Plyvel installation on Travis
Remove Redis. cf #17
Add get_fid in utils

Now the fid is determined from the file name. It's a unique
identifier created via a UUID.
Use sessions in Escalator client.

Add get_escalator_uri in utils.
Merge get and get_default in Escalator client.

If a default keyarg is passed to get, it will be
returned if the key doesn't exist.
Fix thread-safety in WriteBatch
Add empty directory for stocking the databases.
Add logging to Escalator
Update the launcher in order to use Escalator

- Escalator is launched instead of Redis
- The session is now given as an argument to the drivers
  and the Referee
- The rules and the options are serialized with Escalator
- The parsing of the setup has been extracted from start_setup
Update the Referee in order to use Escalator

- Each event is now a key/value (fid, source) pair prefixed by
  'referee:event'
- The events are sent to the drivers using the new scheme
Update Metadata in order to use Escalator

- New 'extra' argument for entry's attributes
- Remove 'revision' attribute
- Update the doc
- Initialize all the properties in __init__
- Use the new fids
- Use a hash for the properties
Update LocalStorage to use the new Metadata