Token locking

It is now possible to lock tokens by setting the timeout propery of the
Pool. (Auto)refreshing of locks is not yet implemented.
Pydoc documentation

Most methods now have (short) documentation in pydoc format.
Remove class variables

The class variables are not needed because we are using instance
variables. Also remove the underscore prefixes to improve readability.
Lock autorefreshing

Implemented lock autorefreshing using the threading library. We use a
separate thread for every lock.