Bug 990834 (part 1) - minor refactor of hawk tests. r=ckarlof, a=sylvestre
Bug 990834 (part 2) - Add support/tweak retry and backoff header support to hawk and tokenserverclient. r=rnewman, a=sylvestre

Conflicts:
	services/common/hawk.js
Bug 990834 (part 3) - Fix handling of hawk errors. r=ckarlof, a=sylvestre

Conflicts:
	services/fxaccounts/FxAccounts.jsm
	services/sync/tests/unit/test_browserid_identity.js