Set HTML class attribute of created knob and wheel child elements

[ul-gh]

Set the HTML class of the created knob to "knob_dial_knob" and that of the wheel to "knob_dial_wheel" to allow styling of any number of JogDial.js instances with a common ruleset.

This makes it much easier when using JogDial.js inside a re-usable or auto-generated component, where it is difficult to automatically assign a HTML id, which by definition must be unique on the whole web page/application.

Because the JogDial.js controls have a unique JavaScript representation in any case, and the base element can be supplied from a number of different methods other than getElementById(), it is also not necessary to have the unique HTML id set.