adding possibility to disable the pulse

As discussed in #8.

There might be some refactoring left to do. I'm not really happy about the TkGui configuring the ColorPicker based on the pytddmon object (but it's still OK with demeter's law).

Moving ColorPicker outside of the TKGui also doesn't sound very appealing because the color is very much GUI related.
Maybe the ColorPicker really belongs in pytddmon since it's dealing with test statistics. But that would violate demeter's law (tkgui should not access the colorpicker through pytddmon).

Any ideas or pointers?