Make DateRecordValue timezone aware

The timestamps it produces are now in UTC by default. Any other
timezone can be used by passing one with the timezone __init__ param.
Generify DateRecordValue gets the timestamp value
Add ipython notebook example of using jsonlogging
Fix json.JSONEncoder not having encoding kwarg in Python 3
Require six for Python 3 compatibility
Remove reference to basestring for Python 3 compat
Wrap keys() calls with list() for Python 3 compat

keys() returns a view object rather than a list of keys in Python 3.
Reference StringIO via six for Python 3 compat
Don't hardcode ZeroDivisionError's module

It's different in Python 3.
Add six to requirements.txt