Adjusting the switchbutton plugin (Bug #1396430)

1) Stop the callback function firing onload of page

2) Allowing to specify the function by reference so one can have either:

  callback_on: function() { },

or

  callback_on: "my_function",

3) Allowing the hidden checkbox to still exist in the tab index for
screenreaders

Change-Id: Ic7a153083446be89eca9f6457b3e05e69cb55a4b
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>
Adding the switchbox pieform element (Bug #1396430)

This allows one to add a pieform element 'type' => 'switchbox'
to render a checkbox styled as a toggle switch.

See file for list of arguments that can be passed to switchbox

Change-Id: I446b676326a9d5e756d5ff645761d8daf27222b5
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>