Bug 915312 - Part 6: add Scrypt and PBKDF2 interfaces.

This also adds testing only implementations backed by SpongyCastle.
Bug 905433: First version of PICL key stretching.
Bug PRE: add BaseResource.post(JSONObject).
Bug PRE: add HKDF.derive helper.
Bug SRP: Add SRP constants.
Bug SRP: add SRPConstants.hexModN helper.
Bug SRP: implement FxAccount.