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 916316 - Part 1: move MILLISECONDS_PER_ constants to GlobalConstants.
Bug 916316 - Part 2: extract DateUtils.
Bug 916316 - Part 3: reimplement date formatting, allow memoization of day formatting. Benchmark tests.
Bug 916316 - Part 4: memoize day formatting during document generation.
Review comment: introduce DateFormatter rather than memoizing.
Review comment: remove additional millisecond constants.
Review comment: rename getDateStringFormatter to getDateStringUsingFormatter.
Review comment: final.
Review comment: use a single DateFormatter throughout HealthReportGenerator.
Review comment: remove static getDateString.
Follow-up: truncate StringBuilder inside Formatter.
Bug 916316 - Speed up FHR date formatting. r=mcomella