2.1 utc date in iso8601

* rake new_post and new_page now use UTC for the file name and dumps an full ISO 8601 datestamp in utc for date field
* rake update_dates adjusts existing dates in pages/posts to ISO format and UTC. it assumes they are in the current local timezone if it's ambiguous. optionally, a timezone can be specified.
* _config.yml has an additional entry for specifying a display offset used on site generation.

There are two gotchas with this code. First, there's no easy way to deal with daylight savings time. I can do it with the TZInfo, but that adds another dependency. Second, while the update_dates task adjusts dates within posts, it does not correct the date in the filename. I wasn't sure if changing filenames was wise or not, but ideally the date in the filenames would all be in UTC.