Added -S support. Need more tests.

src/zc/buildout/isolation.txt

+Isolation
+=========
+
+When developing applications, we recommended that you work with a
+minimal Python environment and let buildout install needed
+dependencies.  If you share a Python environment with other
+applications [#simple]_, you're likely to have unexpected
+interactions with the other applications, through extra modules or
+module version clashes.
+
+There are a number of ways to achive isolation from other
+applications:
+
+1. Use a "clean" Python installation that doesn't contain any
+   non-standard modules.
+
+2. Use a 'virtual environment
+   <http://pypi.python.org/pypi/virtualenv>'_ that wraps a Python
+   installation in a way that hides 3rd-party modules.
+
+3. Use the -S Python option when bootstrapping buildout.  This will
+   bypass the normal execution of the Python site.py file, which is
+   what usually causes directories containing third-party modules to be
+   added to the Python path on most platforms [#macos]_.
+
+   If the -S option is used when bootstrapping buildout, then it will
+   be remembered by buildout and any scripts generated by buildout
+   will also use the -S option.
+
+The first option works best in our opinion because, in addition to
+isolating you from extra third-party libraries, it protects you from
+other changes that a preexisting build might have, such as missing
+standard libraries or non-standard Python build options.
+
+This document tests the 3rd option.
+
+    >>> sample_buildout = tmpdir('sample')
+    >>> cd(sample_buildout)
+    >>> import sys
+    >>> print system("%s -S %s init demo" % (sys.executable, bootstrap_py)),
+    ... # doctest: +ELLIPSIS
+    Downloading http://pypi.python.org/packages/source/d/distribute/...
+    Creating '/sample/buildout.cfg'.
+    Creating directory '/sample/bin'.
+    Creating directory '/sample/parts'.
+    Creating directory '/sample/eggs'.
+    Creating directory '/sample/develop-eggs'.
+    Generated script '/sample/bin/buildout'...
+    Getting distribution for 'zc.recipe.egg'.
+    Got zc.recipe.egg 1.3.3dev.
+    Installing py.
+    Getting distribution for 'demo'.
+    Got demo 0.4c1.
+    Getting distribution for 'demoneeded'.
+    Got demoneeded 1.2c1.
+    Generated script '/sample/bin/demo'.
+    Generated interpreter '/sample/bin/py'.
+
+If we look at the scripts generated, se see that the -S option is
+provided to the Python executable:
+
+    >>> cat('bin', 'buildout') # doctest: +ELLIPSIS
+    #!/usr/local/bin/python2.7 -S
+    ...
+
+    >>> cat('bin', 'demo') # doctest: +ELLIPSIS
+    #!/usr/local/bin/python2.7 -S
+    ...
+
+    >>> cat('bin', 'py') # doctest: +ELLIPSIS
+    #!/usr/local/bin/python2.7 -S
+    ...
+
+
+
+
+
+
+.. [#simple] It's worth noting that system Python builds can make
+   simple applications easier to build, as hard-to-build extension
+   modules are already installed or can be easily installed using
+   system packaging tools.
+
+.. [#macos] On Mac OS X, there are some directories containing
+   3rd-party libraries that are added to the Python path regardless of
+   whether site.py is loaded.  Still, even on Mac OS X, use of the -S
+   option prevents some 3rd-party modules from being added to the
+   Python path,