Outbailment id sort

This PR adds the sorting of []*OutputReq by OutBailmentID. It also adds explicit modelling of the OutBailmentID as an 'sortByOutBailmentID' seemed confusing without any definition of what an OutBailmentID was.
I'd rewrite those two as one-liners, and do away with the comments. If we change the comment of byOutBailmentID to say be like "byOutBailmentID defines the methods needed to satisify sort.Interface to sort a slice of OutputRequests by their OutBailmentID", I don't think we'll lose any clarity. And that seems to be the standard way of defining those sorting methods in btcwallet