fix deps: provide cssselect required by recent versions of twill
force use of twill 0.9

we have build errors with jenkins, just pin it to a known working version
added session_repository

session_repository is a service above repository. It's aware of session transactions (commits/rollback)

Conflicts:
	abilian/app.py
session_repository: there maybe more than one session object living in the application.

Interface changed to take this in account.
app: fix exception handling when session is not usable
indexing: fix auto whoosh schema

it was using columns name for model attributes, whereas they may
differ (like when using a very long attribute name and shortened one for
db column)
update flask-mail to current git version (fixed commit), set 'Sender' header in outgoing mail and smtp envelope

(cherry picked from commit a9c8b71f7a7989ab21caf8c816d1f00ac62f9f21)

Conflicts:
	abilian/core/extensions.py
celery: install sentry handlers
tox: generate junit xml
repository tests: cosmetic changes
repository service: remove global logger instance
session repository: create transaction directory only after "begin", not at instanciation time

Conflicts:
	abilian/services/repository/service.py
repository: fix when using sqlalchemy's subtransaction (!= nested)
fix date/datetime fields not supporting years before 1900