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.
Convert all floats to BigDecimal

The changes made to values in the tests are actually corrections from
previously inaccurate calculations due to floating point storage.  They
would have been potentially inaccurate on different architectures, and
did not represent the actual underlying math.
Coerce inputs to BigDecimal to_s

In ruby 1.9.2, BigDecimal didn't handle being passed a non-string for
its convenience function.  This makes the BigDecimal change support
1.9.2
Require 'bigdecimal' only where necessary
Merge pull request #42 from knewter/feature/grab_more_order_data

Fetch more details from the orders api