Addresses buildout issue #165: Re-add the ability to work offline.

Notes:

- I added the option "--setup-source" to use as the location from
  which ez_setup.py will be retrieved.

- I decided to re-use "--find-links" to supply the download_base
  argument to ez_setup.use_setuptools(). This option is already being
  used to supply the -f parameter to easy_install. I don't think
  there's much point in having 2 different options: one for
  use_setuptools() and one for easy_install itself. buildout 1 used
  the option name "--download-base" for this functionality. But
  buidout 2 already uses "--find-links". I think keeping with
  "--find-links" and not resurrecting "--download-base" is the best
  option.

- I used the short option name "-s" for "--setup-source". I'm not sure
  if this is desirable, or if we just want to skip a short option
  name. buildout 1 did not have a short option name for
  "--setup-source", but buildout 2 does use "-f" as the short name for
  "--find-links". So I think that for consistency with other buildout
  2 options, "--setup-source" should have a short option name.

- I did add one slight change in behavior: I always make sure that the
  argument to "--find-links" now contains a trailing slash. This is
  needed when I pass it to ez_setup.use_setuptools(). By making this
  change, I've ensured that if find-links is provided, easy_install
  will be called with a "-f" argument that ends in a trailing slash. I
  don't think this is important, because easy_install works with or
  without the trailing slash.

- I have not yet added any documentation or tests.