append the "+0000" offset to utc dates if specified

We, unfortunately, have some code out in the wild that doesn't parse utc dates unless they have the "+0000" offset at the end. I added a config option that will optionally add this offest for utc dates.

I've also added some unit tests and verified the current tests all pass:
650 passed, 0 failed, 12 skipped (see 'Task List'), took 5.23 seconds (NUnit 2.6.1).