Better cite-completions

This is a rather large overhaul of the citations-completion system. I changed things from working with regular expressions to use pybtex's parsing engine, which is much more feature complete. I realise this might be too big of an overhaul to get in and the bundled external dependencies (pybtex, pkg_resources, and pyparsing) makes it a bit less than ideal, but I figured I'd flag this for anyone who is interested.

Features:
* Unicode (utf-8) support
* Translates LaTeX special characters into proper unicode characters
* Adds some (very basic) support for crossrefs (i.e. it will pull author, title or journal information from a parent record)

Additionally, and mostly by accident, it resolves #418, #407, and #237, mostly as pybtex is more robust than the current citations-completion.