Ledger test

https://ripplelabs.atlassian.net/browse/RIPD-432

@JoelKatz @vinniefalco 

This is still somewhat of a WIP, but it is time to get the bulk of it onto develop, get some experience with it, and see where we go from there.
This doesn't really sign the transaction, so much as fill in the public key which allows the STTx constructor not to blow up after the format is applied.