BearerTokenModule_addNewToken allows to add token for Person passed as
destination_reference. In case if Person has no key assigned new one is
generated.

BearerToken_isValid allows to validate token against (possibly) related person
object.

Base_getHMAC is low level interface to hmac module.

In order to made tokens unique use User-Agent and REMOTE_ADDR in token
generation body. Thanks to this token will be narrowed to one broswer. In order
to avoid reuse token on different servers add Host.

Key to generate HMAC are stored on Person object using bearer_token_key property.