Support for arbitrary number of days of back history

This implements an ability to tell the client how many back days of trading history to get at startup.  In addition, this should deal with long periods of connection failure by pulling trades sequentially up until the current time.

I mainly wanted to see if you'd be interested in this type of patch, which would be useful for those who want more history in their strategies.

Additional considerations:
- Takes a while to download trades, probably should have some way of caching them (maybe cpickle).

- For RAM considerations, one might want to consider garbage collecting candles over a certain age for long-running instances, but this could be a separate patch.