Update color button - clearing color/background-color property

I updated color dialog with below features:

 - Add `Transparent` for clearing background color.
 - Add `Reset to default` for clearing foreground(text) color.
 - Shape up color dialog.

This patch will solve https://github.com/HackerWins/summernote/issues/76

After applying this patch, color dialog will look like below image.
![screen shot 2013-10-18 at 2 36 38 pm](https://f.cloud.github.com/assets/579366/1358488/aafa1fa6-37c3-11e3-8f08-0f4ce8c6c81f.png)