Support fxos v1.3+ mozMobileConnections vs old mozMobileConnection.

As discussed in the v1.3 release notes:

  http://www.mozilla.org/en-US/firefox/os/notes/1.3/

The `navigator.mozMobileConnection` property has been renamed to `navigator.mozMobileConnections`. (Note the trailing 's'.)  This new property is always a DOM list.

This pull request should support both the old style property and the new property.  I have only tested it on my v2.0 flame device, however.