Requirement cleanup + Store everything as string

Hi,

I made multiple changes:

- I cleaned up a bit the dependencies. Now missing requirements don't lead to a failure. I also cleaned up the Rakefile and the tests slightly, reducing the number of hard deps.
- I included the parser.rb in the repository. I prefer to include generated code in the repository to be independent of the racc version. I think most projects do it similarly. You also don't need racc to start developing then.
- I made modifications to store everything only as string. This change is backward incompatible and a matter of taste and style. You could pull it when you release the gem as bibtex, then the incompatibility wouldn't matter that much. But then you couldn't just depend on it.
I tend to use symbols only for few things where the set is limited. I think this style is also more common in most other libraries (e.g. XML libraries with DOM etc)

Looking forward to your response :)

Daniel