MAINT: Check attributes instead of contains for event fields.

In support of source that emits a subclass of Event which defines some
fields as properties instead of doubling the value in the
`Event.__dict__`

Use hasattr instead of the overridden __contains__ method of the Event
class, so that when non-algorithm facing code checks for field existence,
properties count.

Intentionally not touching the `__contains__` in Event, to avoid
changing, at the moment, any algo behavior that relies on the
`__contains__` behavior's use of `__dict__`
Merge branch 'master' of https://github.com/Quantopian/zipline
Reworking of the futures source grabber, now pulls data into a panel.