Adapt transaction, trade and order objects.

Add a common file and a common quantize method
Fix file indentation
... what a pain to merge :)

I've adapted the Order and Trade classes so that they match the design of the Transaction class (`time` is now `datetime`, consistent in all classes)


I forgot to stip out the changes to `Cryptsy.get_my_trades()`. The method has an undocumented limit of 200 trades and there is a second one that allows to filter trades by market on server side. This patch fixes the limit problem and implements the filter function.