Add synctex support

Hello,

I added Synctex support to LatexBox.
Currently, it only works for evince (only forward search, proper support requires DBus which is too involved for me) and zathura (also backward search).
I also plan to add support for skim on OS X, which I use at work, but this will have to wait until I can test it on the mac. I have not written proper documentation yet either.

Please let me know what you think of this patch.
I am not very happy about the backward search implementation, which needs a whole global function to work around the fact that synctex writes the file paths like so `/path/to/file/./document.tex` which `vim` does not like. Maybe it is better to write a small bash script to take care of this and use the standard `vim --remote +line document.tex` instead?
There is already a small wrapper script to call zathura, I tried without at first but ended in a nightmare of shell escaping. If anyone manages to make it work directly in vim script, it would probably be better.