Bug STRETCH: first version of PICL key stretching.

This version is built on PBKDF2 and scrypt.  The algorithm is
described at
https://wiki.mozilla.org/Identity/AttachedServices/KeyServerProtocol#Client-Side_Key_Stretching.
Bug PRE: add HKDF.derive helper.
Bug SRP: add SRPConstants.hexModN helper.
Bug SRP: implement FxAccount.
Bug PRE: add BaseResource.post(JSONObject).