add Item&setContent() for Atom feeds

- Make it easy to get a <content> element on Atom entries
- Also spells "enclosure" correctly - "setEncloser" -> "setEnclosure"
- Fixes Atom test file on case-sensitive filesystems
- Tack a date_default_timezone_set() on tests, to avoid warnings
Merge pull request #11 from sparkfun/master

Add setContent() for Atom feed items