auth: add function for application token generation

Related to #388.
auth: added CheckApplicationToken function

This function will check the whether a given token is valid.

Related to #388.
auth: don't mark user tokens as valid application tokens

Related to #388.
auth: implement json marshaling of tokens

Related to #388.
api: starting to build generateAppToken handler

Related to #388.
auth: validating the presence of the client key in the JSON body

Related to #388.
api: fix HandlerSuite setup
api: unexport handler and authorizationRequiredHandler
api: added adminRequiredHandler

This handler should be used to wrap handlers that _require_ admin users.