Bug 835793 - Fix errors in the list of modules supported by Fennec

- Updated `widget` as unsupported module
- Added `page-worker` and `system` as supported module
- Added Flavored MD syntax for deleted content (~~)
- Updated `sdk-docs.css`
- Removed comments from `mobile.md` to make it more readable

I added the Flavored MD syntax used in github: in this way, I was able to remove all the HTML used previously, and the md file is more readable and easier to maintain. Also, is displayed on github as well (e.g. https://github.com/ZER0/addon-sdk/blob/3c0f7cfbc7302c365bbd854c23d81e8f9016f2d1/doc/dev-guide-source/tutorials/mobile.md)