Time parsing.

Rather than just using a string for PubDate, we attempt to parse it.
This includes a couple of crazy non-standard time formats that I've seen
in the wild.

Breaking change: Item.PubDate is no longer a string, it is time.Time.
Back to previous API.

(IE, the previous breaking changes are unbroken.)
go fmt
Merge pull request #37 from sirsean/parse-time

Time parsing.
Merge branch 'master' of https://github.com/jteeuwen/go-pkg-rss into database-interface