Revision of installation instructions

This builds on what is already in PR #27.  The only new thing here is that I've modified the installation instructions page substantially:

 - The actual install commands appear at the top, so most users won't need to read the whole page
 - The full install involves grabbing a tarball of the latest release
 - Separate instructions are given for a PyClaw-only install
 - The section on running an example also includes how to run a PyClaw example.
 - There is no longer any need to set the PYTHONPATH.

I'm not sure everyone will agree with the new page, so please take a look and feel free to criticize.

Obviously, the tarball link will need to be updated to the actual release tarball.