1.  Putting common install instructions in a single include file `_includes/setup.html`.
2.  Modified Makefile to make `index.html` and `setup.md` depend on the new include file.
3.  Got rid of the old piecemeal setup instructions.
Merge pull request #316 from gvwilson/refactoring-setup-instructions

Refactoring setup instructions to eliminate duplication.