Add initial document type declaration handling

[ljedrz]

For now it only supports the SYSTEM variant without the optional intSubset. I only started working with this code today, I'm open to any criticism and modifications.

[shepmaster]

Thanks!

[shepmaster]

I'm assuming you ran into this with a real-world usecase, does this PR by itself help solve whatever problem you are facing, or do you think there are more coming? Asked another way: would you like to see this released?

[ljedrz]

Yes, I encountered this with documents that had a DTD specified - they caused panics without it. I think this implementation covers the most popular case (when you just want to ignore the DTD and parse the actual content) and could be helpful on its own, but I might hit some other unimplemented cases after Christmas if you would like to pull some more code before doing a release.