Added convenience methods Rika.parse_content, Rika.parse_metadata, and R...

...ika.parse_content_and_metadata for (what I think are) the most common use cases.

Richard, I've found that most of the time I only want the content or the content and the metadata, and thought these convenience methods might be useful for others as well.  What do you think?

If you want this change, I can add explanations of this to the README if you like.

I'll be doing a lightning talk on JRuby at BigRubyConf in Dallas, Texas this Thursday or Friday and I'm planning to show Rika. I was thinking it would be really cool to be able to parse a document in a single function call.

Cheers,
Keith