feature: added a Filter module.
feature: got rid of (Maybe OrderBook) in pipe chains.
feature: also export fulldepth from HttpApi.
Merge branch 'initOrderBook'

Conflicts:
	src/Data/OrderBook.hs
bugfix: lost a line in the merge.
feature: here's a take to solve the initial order book problem.

I'm not sure, if it works, since I hardly ever see messages buffered. If nothing
gets buffered, it certainly works.