Add time check to validation.

The client and controller both check that the time specified in the
date header is no more than 15 seconds old, and no less than 0 seconds
ahead of the current time. This should prevent replay attacks (as the
date in the header is part of the signature).

Signed-off-by: Tyler O'Meara <Tyler@TylerOMeara.com>
Merge branch 'feature/preventReplays' into develop

Conflicts:
	brave/api/client.py
	brave/api/controller.py