BUG: Stops the TradingEnvironment from being created at import time when
zipline.utils.events is imported.

Changes the class level attribute `env` on EventRule to a property so
that the environment is only looked up at when needed.
MAINT: Remove use of deprecated getchildren method on xml element

Rather than calling getchildren on xml.etree.ElementTree elements,
we're now supposed to just itegrate over the elements.
BUG: Fixes various bugs with the event manager:

- NotHalfDay only worked at midnight
- week_(start|end) were actually month_(start|end)
- Removes check_args from api.
- Default offset of 30mins for market_(open|close)
add account object to context