Fetch more details from the orders api

There were some Mt. Gox Order Details that weren't being fetched into
the local MtGox::Order object.  This change fetches those and makes them
accessible, as well as preferring BigDecimal to Float.

What are your thoughts on these changes @sferik?  I was mildly concerned when I saw floats being used, and so that was an unexpected change I introduced.  The other is really just proxying data through that they provide that we weren't providing.