Only disable keyboard focusing for touch devices [Fixes #1541]

This fixes the issue [1] by first checking to see if the current
device is a touch device.  The other issue [2] that occured because
of the original fix [3] is now fixed, 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].  Information on the reasoning behind this fix
can be found on GitHub [5].

[1]: https://github.com/ivaynberg/select2/issues/1541
[2]: https://github.com/ivaynberg/select2/issues/2223
[3]: https://github.com/ivaynberg/select2/commit/d87e93dd45ade99e7894ac4854bf65e8f59667d4
[4]: http://stackoverflow.com/a/15439809/359284
[5]: https://github.com/ivaynberg/select2/issues/1541#issuecomment-39805859