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.
It'd be nice to finish the conversion of these methods away from `STParsedJSONObject` - or (much preferable) get rid of a lot of the code duplication.  I feel as if reviewing this code is pointless if it's going to be replaced by something different...