Bug 903123: Add HawkAuthHeaderProvider.

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
Bug 903123: Add HawkAuthHeaderProvider. r=rnewman
No bug - Fix some MockHealthReportDatabaseStorage warnings. r=me
Bug 870925 - Part 1: Add constants and SubmissionsFieldName enum.
Bug 870925 - Part 2: Add init functions and UploadRequestDelegate.
Bug 870925 - Part 3: (WIP) Record FHR submissions data.
FOLD ME.