RealtimeClock: wall time based clock for live trading

[coveralls]

Coverage decreased (-0.03%) to 87.713% when pulling a6e935e6cf2fe05f7e56b9a1a8e07538a98a9599 on tibkiss:feature/realtime_clock into 11ba5f503f9b6758c6f26893914da1ade94b330c on zipline-live:live.

[pbharrin]

the build fails on Python 3.4, because it cannot import izip_longest.
https://travis-ci.org/zipline-live/zipline/jobs/232577596#L3781

I am looking into a Python 3.X replacement for this.

[pbharrin]

I found a fix: izip_longest is called zip_longest in Python 3.X http://stackoverflow.com/questions/38634810/failing-to-import-itertools-in-python-3-5-2

[tibkiss]

Thanks for merging. I'll enable compatibility checks in idea to avoid such problems in the future.