Sublime text 3 support

I've tried to push ST3 support a bit further. 

The main issue concerns paths handling and module loadeing. 

The Loader object is initialized with os.getwcd(), which on my system is /opt/sublime_text. This causes SublimeLinter to not find any module, and make it quite inert.

I tried fixing this by using sublime.packages_path() as a basedir instead, and changing the relative module path by adding SublimeLinter as a prefix.

This allows modules to be found and imported.

After that, I've fixed a few python3 related issues, used 2to3 on capp_lint and updated pyflakes to the latest, python3 compliant version.

The last issue is that sublime.packages_path() isn't defined early on, during sublime's init.  So I tried using set_timeout_async to delay the loading of modules.

This makes SublimeLinter functional again for me. I've only tried it on python code though, so I cannot say much about other linters.