headings atop only the main textblock, not across the
  textblock+sidenotes.


git-svn-id: https://tufte-latex.googlecode.com/svn/trunk@64 516e2f36-ce3a-0410-bea4-1d4a03f5df72

* Created a new tufte-book document class.
* Moved most of the tufte-handout code into a tufte-common.sty file.
* The tufte-handout.cls and tufte-book.cls files now include
  tufte-common.sty.
* No documentation for the new stuff yet -- read the code.
* The tufte-book class is very much unfinished -- I've only just started.
* Bugs abound!


git-svn-id: https://tufte-latex.googlecode.com/svn/trunk@63 516e2f36-ce3a-0410-bea4-1d4a03f5df72