Replace `webbrowser` package with native calls to `run_command('open_url', ...)`

When using the python webbrowser module, the plug-in opens the wrong browser (Firefox instead of Chrome), on linux.

Replacing the calls to `webbrowser.open_new_tab` with the built-in sublime API call `sublime.active_window().run_command('open_url', ...)` seems to correct this problem.
Merge pull request #2 from rarescosma/master

Replace webbrowser.open_new_tab with native sublime API call to fix browser inconsistencies