Checking python version when specified.

The [versions] section is checked for a 'python' key.
Functionality copied from buildout-versions.
Includes documentation and tests.
Added show-picked-versions option (default false).

Included: functionality (copy/pasted from buildout-versions) to
actually record and print the picked versions. For this, two new
class-level attributes on the Installer are added.

TODO: testing whether the printing of what required what also
works.