Document SF namespace

[oleq]

SF (since #34) should be documented as a standard component, with methods listed the same way variables and classes of other components have already been documented. Most of the methods are self-explanatory and correspond with CKEDITOR* methods, so we don't need to waste time for detailed documentation (arguments, returns, etc.) right now. So:

• What is SF.
• List of methods.
• A simple HTML snippet that explains how and where to put sf.js file.

Related #35.