Fix issue30 color picker pushbutton not work

This issue is related to pixmap based style.
This style only use on Windows and MacOS, thus only these platform run into these problem

The detail can be found here:
http://qt-project.org/faq/answer/why_does_nothing_happen_when_i_set_the_palette_background_color_on_a_qpushb

To solve this, I use stylesheet property instead and solve this issue.
However this cause a little redundant code since we cannot get color in stylesheet.
Merge pull request #69 from yodalee/issue30-diagram-colorpicker-not-work

Fix issue30 color picker pushbutton not work