In Mac OS, show previews in browser instead of HTML program

In Mac OS `open` with a path to an HTML file will invoke the default program for opening HTML files. For me and many Sublime users, this is Sublime, not Chrome.

This adds code which automatically gets the default browser on Mac OS.

Here's a Stack Overflow question I just posted, on the subject: http://stackoverflow.com/q/23506033/893113
Default browser: Fixes for Sublime Text 3
open_in_browser: Correctly honor browser setting
Merge pull request #194 from paulmelnikow/default_browser

In Mac OS, show previews in browser instead of HTML program