Added a style guide and some suite design sections to the CUG.

This adds a new "Style Guide" section, and three new subsections at the end of "Suite Design Principles", to the user guide. 

@dpmatthews et. al. - could you review this at your leisure and let me know if you disagree with any of my suggestions, or if you have additional suggestions.  This material is largely cut-n-paste from a guide I knocked up  for local collaborative suite development (I have a user or two with rather radical opinions on code styling :-).