use browse-url function instead of browse-url-default-browser

The proper way to browse an URL from Emacs is to call the `browse-url' function
that will eventually use the browse-url-default-browser.  This is important
because browse-url is in the autoload while browse-url-default-browser is not.
Merge pull request #23 from jeremy-compostella/master

use browse-url function instead of browse-url-default-browser