Store session values in cookies.

Currently, we (or more precisely, beaker)  store session values in the filesystem.
That does not scale, and is extremely hard to synchronize across multiple machines.
Therefore, this code adds a new flag adhocracy.session.implementation that can be set to "cookie" to use a new session implementation which stores the session data in cookies.
The current implementation does not encrypt the stored data (as I haven't found any secret values), but does check a signature in order to prevent users from meddling with these values.
PEP8 improvements
Merge branch 'develop' into cookie-session
Check for empty keys as well
Only use a single cookie for session data
Check session expiry in cookie-based sessions
Default session lifetime to an integer

If a year seems overly long, consider that github uses 10 years.
Clean up cookie session implementation
Correct setting and deleting of cookies
Merge pull request #361 from phihag/cookie-session

Store session values in cookies.