Adjusting Whitelist behavior

Adding a single point of lookup for who's on a whitelist (as Admins
should always be whitelisted). Added a few tests to verify behavior.

Adjusted Admin constraints to provide a general "is_admin?" method.

Added a controller to allow for maintenance of whitelists. Added a
base admin controller to provide URLs to helpful functions.
Merge pull request #66 from ndlib/adjusting-whitelist-behavior

Adjusting Whitelist behavior