Only disable keyboard focusing for touch devices

This fixes #1541 by first checking to see if the current device is a touch device.  This also fixes #2223 that occured because of the [original fix][3], because the hidden inputs are always focused by default on non-touch devices.

The code used for detecting touch devices was [pulled from StackOverflow][4].

[3]: https://github.com/ivaynberg/select2/commit/d87e93dd45ade99e7894ac4854bf65e8f59667d4
[4]: http://stackoverflow.com/a/15439809/359284