Added notes on what to do for buildout-versions integration.
I need to go through buildout-versions' finish() method, still.
Added notes on what to do for buildout-versions integration.
Everything is in place now for actually implementing it.
Update for show_picked_versions
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.
Added doctest normalizer for distribute version
Added test for not printing versions if nothing has been picked.

Possible problem: distribute is also reported as have-to-pick. So I
pinned it to 0.6.34, the current value. It might bite us later on, I fear.
I don't know enough about the setup to be sure.
Added test for who required which picked version
Added normalizing of distribution names in [versions]

The Python package index is case-insensitive. Both
http://pypi.python.org/simple/Django/ and
http://pypi.python.org/simple/dJaNgO/ work. And distributions aren't always
naming themselves consistently case-wise. So all version names are normalized
and case differences won't impact the pinning.
Refactored code duplication
Using print() function instead of print keyword

I cannot handily use print_ from buildout.py as I get a circular import that way.
Moved picked version printer so we can use print_()
Got printing of picked versions to file working.

An 'update-versions-file' option points at a file you want updated.
Updated changelog
Fixed test: added the new update-versions-file option.
Fixed typo
Cleaned up comments.
Raising an error if buildout-versions or buildout.dumppickedversions is used
Removed exclusion of everything except the master branch.

I guess this was why the pull request wasn't build.
Removed the python version checking.

As discussed on the mailinglist: better to have python version checking
as a separate buildout option instead of hidden in the [versions] part.
And... the python version checking itself needs to be more robust.
sys.version is too long, minor/major needs explicit checking.