Touch events and mouse button selectors.

First crack at supporting touch screen for devices like Android and
iOS tablets. Part of https://github.com/kanaka/noVNC/issues/48.

This change detects touch screen support and uses the touchstart,
touchmove, touchend events in place of the normal mouse events.

In order to support middle and right mouse clicks, if the device is
a touch device, then three toggle buttons are added to the UI
representing the left, middle and right mouse buttons. These select
which mouse button will be sent when the screen is touched. All the
buttons can be toggled off, in which case then the touch events only
move the mouse cursor rather than sending a mouse down and mouse up
for touchstart and touchend events respectively. This allows fairly
full control with the mouse on touch screens.