Documentation and style fixes/improvements.

Stuff changed, from commit msg

- param as_objects in request_json has been rephrased to keep it within
   one line as the automated documentation creater has problems if the
   final param (in the generated order) streches over multiple lines.
 - Change __init__ docstring to reflect that it is the docstring for the
   PRAW package, eg what people get when they help(praw).
 - Add link to Reddit.com's feedback page to 'send_feedback'.
 - Put the summary part of the docstring on it's own line for consistency.
 - Modify README so the first three sentences doesn't start wiht 'PRAW',
   use the word 'designed' instead of 'coded' to show following API rules
   is a design decision, i.e something which won't change.
 - Added that downloading from github or checking out the code may
   return unstable development code.