Fixes a crash in various transforms when providing CUSTOM events whose fields
don't match the fields required for the transform.
This is fixed by requiring all `EventWindow` subclasses to supply a `fields`
property, which returns a list of strings that are required keys for any event
that can be processed by the window. Any CUSTOM events the don't supply the
required fields for a transform window are ignored by that window.
The WrongDataForTransform was referencing a `self.fields` member,
which did not exist.
Add a self.fields member set to `price` and `volume` and use
it to iterate over during the check.
Use six's with_metaclass to have objects that use metaclasses, in
both Python 2 and 3.
Otherwise, in Python 3 the objects were being treated as if they
did not have a metaclass, when the Python 2 syntax is used, leading
to errors because of missing attributes, etc.
So that algorithms that specifiy market_aware, days and delta
as args can be transitioned to just specifying a window_length kwarg.
Moving towards removing market_aware and delta completely.
Scott Sanderson