"Step" Function

Added support for a "step" tag so that users can add 'step="val"' to the
html tag to set the value to step by.
Example:
<input type="text" data-role="spinbox" name="spin" id="spin" min="0"
max="500" step="20" />
The '+' and '-' buttons will now increment the value by 20. (the default
is still set to 1)