added the possibility of customizing the color of different parts

I added two options : "monochrome" a boolean which is true by default and "colors", an object to configure the colors of different elements like sun, cloud etc..

When you set monochrome to true, icons are colored like this 
![skyiconspreview](https://cloud.githubusercontent.com/assets/711050/3562647/2876644c-09fd-11e4-9c76-67d30791a5c3.jpg)

You can pass custom colors in the options. I also edited the README to show how to configure this new feature