This fixes #80 - make it work on remote files

This commit is the first draft of a PoC on how it is possible to have syntastic
work with remote files (so bcvi + netrw + syntastic = win!)

Feedback welcome!

I've never written nor hacked on vim plugins before, so it is ugly, but works.
I'll be more than happy to change the code to fit your requirements.
I think one thing we could easily do is check if the file is readable, then do nothing, and if it isn't, pass it through a regex to see if it's potentially a NetRW, and then use my temporary file (+ rename) trick.