Fixes Issue #2. Now works under OS X *and* Linux.

At the cost of creating a new, "internal" function (`_opengit_open`), tests the `$MACHTYPE` environment variable to determine if running under OS X. If so, uses the command `open` to open the Github URL; otherwise uses the (previously hard-coded) [`xdg-open`](http://linux.die.net/man/1/xdg-open) command.

Tested under OS X (duh; 10.9) as well as Linux Mint 15.