Added Colorpicker metabox field

This incorporates minicolors.js color picker:
[http://labs.abeautifulsite.net/jquery-minicolors/](http://labs.abeautifulsite.net/jquery-minicolors/)

There are options for opacity as well, which would be great to add to the settings array like:
`opacity: true;`

But saving and outputting rgba values is still something I haven't figured out.