Update to create as well as consume Atom feeds

[csells]

• provided AtomFeed.toXml function
• added tests for XML generation
• added tests for RFC 5005 (Atom pagination)
• updated Atom.xml and Empty.xml to pass ATOM feed validator
• updated the example app to show off feed generation
• tightened the Atom OM types via use of Uri and DateTime
• updated Dart code to use less code
• updated README

Did not generate Dublin Core or Media extensions to Atom.

Update RSS code a little (and made sure that the tests continue to pass) but have not provided feed generation for RSS; I plan to do so in the future.