Requirement cleanup + Store everything as string

Hi,

I made multiple changes:

- I cleaned up a bit the dependencies. Now missing deps 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 with the name bibtex or as version 3.0, then the incompatibility wouldn't matter that much. 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/DOM libraries etc)

Looking forward to your response :)

Daniel