Add windows support

Hi! Nice plugin, I wanted to try it but ran into some bugs on Windows.

They were easily avoided, though, so here's a pull request. Things solved:

* `find_mix_project` went into infinite recursion because `/` isn't a Windows path. 
* `mix` is a .bat file on Windows, which you can't just give to Python's `subprocess` like that (which is ridiculous, but that's how it is). The trick is to use `cmd.exe` and suppress the console window that creates.

Note that this is the first time I've ever coded something for Sublime, so if I made stupid mistakes or missed some style guide, please do tell :-)