Implement references management

[pbelmans]

Similar to #4 we have that the proofs (and maybe some statements) will contain \ref's. Right now these are returned by the server as <a href>'s, which of course is wrong (it should be pure TeX).

But what do we want to happen with this? Some options come to mind:

1. turn \ref into a \stackscite to the corresponding tag (every \label in the Stacks project has a tag, right?)
2. check whether there is an existing label in the (local) LaTeX document, if so: refer to this one, if not: goto 1

This also relates to issue #6