Added color listener to ColorPickerView

[MFlisar]

• added color listener to ColorPickerView
• extracted the observable interface into a own class

Hope that's fine for you

[martin-stone]

Apologies for the pause in feedback (real life intrudes)...

Just a few minor tidy ups, then I think it's good to merge. (I'd do them myself, but it looks like the Git branch juggling involved may be complicated and/or clobber the record of your contribution.):

• Could you rename IColorObserver to just ColorObserver -- Java and Android seem to follow a convention of no prefixes on interface names (and I prefer it myself, too).
• Could you put the opening braces on the end-of-line, to conform with the rest of the code.
• There's now an ObservableColor.removeObserver() function that is not used. Let's delete it.
• Could you copy/paste the license header from one of the other files into ColorObserver.java, and remove the "created by..." comment -- Git will record who created it and when.

Thanks.

[MFlisar]

No need to apologise... That's normal...

Made the small changes and pushed them to my branch...