don't fail if filter cannot be loaded
remove latex-decode and multi_json as hard dependencies
add the generated parsers to the repository

- This needs discussion!
- I think it is usually done like this in other projects
- Since the gem also includes the generated file, it is better like this. And then one
  also sees if something really changes in the parser.
- If someone starts developing, he doesn't need racc at first
don't store field keys as symbols
use BibTeX::Symbol instead of Symbol in values
store type as string