Allow init from string
Remove VERSION constant

Requiring the feed_parser.rb in the gemspec means all the dependencies
need to be installed to build the gem, which sucks.

It was also defined as a toplevel constant, which caused conflicts
with other libraries. Not easy to put in a module as there is already
a class called FeedParser, so we can't define a module with that name.

Easiest to just remove it and put the version in the gemspec by hand.
FeedItem: explicitly use self to ensure method_missing is invoked

Fixes using it with Rake, which defines a global link method
Add support for parsing published date
Added ability to instantiate and parse feed through single api call
Added parsing from an XML string to README.
Bump version to 0.3.5.