First version of FxAccountClient.

@rnewman: this isn't quite done, but it's ready for feedback.  I intend to give it a documentation pass, and to factor out, test, and make more strict most of the parsing code.  I don't want to work on this anymore right now and am cycling onto the next ticket :)

Oops, I forgot: I think the FxAccount/SRPSession can be simplified and possibly combined, but I want to write some of the other pieces to see what feels right.