Use README file to describe the bundle.
Ignore config.log from scons.
Move DOC and environment variables to the beginning.
Use $PYTHON to run easy_install.

This ensures easy_install does not run a different python.
Specify explicit python for scons.
Use fixed-version sources for cxxtest.
Use fixed-version tarball for periodictable.
Restrict default NCPU to no more than 4.

Unless overridden in the environment NCPU will be no more than 4.
Add --wipe option for cleaning all downloads.

This reverts the directory to a pristine state.