implement feature-request: Allow color selection for cards

Implement Feature Request: Allow color selection for cards #4

The color will be the same as the color of the label. If no label is given, the default color is yellow.

![trellocolorcards](https://cloud.githubusercontent.com/assets/131087/2973284/6dcd023a-db7c-11e3-9c18-acade74fb568.png)