Add timezone argument for IntervalTrigger

[skoranda]

Since the start_date passed into the
apscheduler.triggers.interval.IntervalTrigger instances is explicitly
using UTC the timezone argument needs to be passed with UTC as a value.
Since apscheduler only supports pytz and not datetime.timezone we pass
in timezone=pytz.utc.

All Submissions:

• [ X] Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• [ X] Have you added an explanation of what problem you are trying to solve with this PR?
• [ X] Have you added information on what your changes do and why you chose this as your solution?
• Have you written new tests for your changes?

It is not clear there exists any test harness for the scheduler yet.

• [X ] Does your submission pass tests?

The same tests pass before and after this change (there are currently 8 failing on master and the 1.1.1 tag).

• This project follows PEP8 style guide. Have you run your code against the 'flake8' linter?

The "new" code is not following flake8 yet.

[coveralls]

Coverage increased (+0.007%) to 68.446% when pulling 34f9b79 on skoranda:utc_timezone_for_interval_trigger into a7eeafb on IdentityPython:master.