added session_repository

session_repository is a service above repository. It's aware of session transactions (commits/rollback)
blob use session repository
deps: added cssselect

required by twill in recent versions. cssselect was previously in lxml
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
Service: added ServiceNotRegistered exception, if_running decorator
fix respository tests trying to call an already instanciated session
session_repository: run event handlers only if service is registered and running
blob: now session_repository requires a session or model instance
fix docstring format
fix download url for progressbar
removed coaster: hard dep on flask-script==0.5.3, and it's decorators are not used
fix deps
abilian.core.models: add symbols from .base
init_extensions and register_plugins are now run with an app_context
fix missing import
csrf: added method to get csrf timeout + simple json view to get current token

Conflicts:
	abilian/web/csrf.py
csrf time_limit: handle case where form value is already an int
register csrf blueprint
assets: setup load path for static_folder