Bug 903123: Add HawkAuthHeaderProvider for Android Background Services. r=rnewman

Hawk is an HTTP authentication scheme using a message authentication
code (MAC) algorithm to provide partial HTTP request cryptographic
verification.  Hawk is the successor to the HMAC authentication
scheme.  The Hawk scheme is descripted at
https://github.com/hueniverse/hawk

Hawk is under-specified; this implementation interoperates with the
git hash at
https://github.com/hueniverse/hawk/blob/871cc597973110900467bd3dfb84a3c892f678fb