Bug 892025 - Pre: add HKDF.derive helper.
Bug 892025 - Pre: Add BaseResource.post(JSONObject).
Bug 892025 - Pre: Fix Utils.byte2Hex(b, length).

I muddled this merging mcomella's byte2hex -> byte2Hex rename.
Bug 892025 - Part 1: Add SRP constants.
Bug 892025 - Part 2: Implement FxAccountClient.
Bug 892025 - Implement Android client for FxAccount auth server. r=rnewman