Save a session subset

Save (job_state_map and desired_job_list) to a JSON file. Added the
necesarry __getstate__() methods to help serializing involved class
instances. Added a SessionStateEncoder to JSON encode the session
instance, adding the __class__ and __module__ values to all serialized
objects. Added the session.persistent_save() method and sync the session
data file using os.fdatasync().
Launchpad automatic translations update.
Increased version to 0.15.2 and moved changelog around a bit
Added JSON encode tests for several classes

Including JobDefinition, ResourceExpression and JobResultTests
Ensure an atomic update of the session file

Rename the __getstate__() methods to _get_persistance_subset()
Keep the session_data_filename at the class level
"[r=zkrynicki][bug=][author=sylvain-pineau] automatic merge by tarmac"