Theme refactoring

This PR is related to: https://github.com/ibm-js/sdk/issues/6
It will have to be merged in sync with its deliteful counterpart to avoid discrepancies.
Here is the picture of the new structure and the type of data we expect in each file: 
![theme_structure](https://cloud.githubusercontent.com/assets/4415497/2543847/f52156f0-b5f7-11e3-9183-391017adfdfc.png)