utc and time parsing and formatting support

Builds on #35 and #30 to add UTC support.

Updated utc-time branch to include current state of master branch.
Removed dependency on times library by adding utcnow, utcformat, from_unix, and to_unix functions, which are cribbed partially from the times library and partially from rq.utils.

RQ actually replaced `times.to_universal()` with [utcparse()](https://github.com/nvie/rq/commit/888d771d4d266e964a4623a432539b8c7a742c66#diff-b460bf5f8f26e7bd8f4cb42060d3d5fdR182), a function that only parses string formatted times. The original `times.to_universal()` can also parse Unix timestamps. That is actually what rq-scheduler was using it for. So, I grabbed that part of the original `to_universal()` function and called it `from_unix()` here.

Seems like the tests passed once I discovered that problem.