Implement market order functionality
Merge pull request #38 from davemarchevsky/market_order

Implement market order functionality
Merge pull request #36 from knewter/doc_typofix

Fix an error in docs, and align some code
Support per-user clients

This makes MtGox usable as a non-singleton by calling client =
MtGox::Client.new.  The older, singleton-esque api still works by
building a client and storing it as a class instance variable on MtGox,
if that's desired.