Buttonwood

Fixed an issue where Price.ticks_behind() wasn't returning a float like it should (but rather a Price). now returns a float.

Added __contains__ to ndeepdict.

fixed another bug with ndeepdict.

Fixed a bug with NDeepDict, which was causing None to not be returned when using .get(<unknown key>).

Fixed some issues with price for better comparison and use.

Buttonwood has been upgraded to python 3. Python 2 will no longer be supported, which means Buttonwood releases < 2.0.0 will no longer be supported.

PriceFactory has been added to Market and Price has been changed to better support math functions and floats.

Use of nosetest has been replaced with pytest.