Merging buildout-versions' functionality into buildout

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.

Drop trove classifiers for Python 2.4 and 2.5

I guess this was why the pull request wasn't build.

An 'update-versions-file' option points at a file you want updated.

I cannot handily use print_ from buildout.py as I get a circular import that way.

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.

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.

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.

The [versions] section is checked for a 'python' key.
Functionality copied from buildout-versions.
Includes documentation and tests.

README.rst claims only 2.6, 2.7, 3.2 and 3.3 are supported.

Everything is in place now for actually implementing it.

I need to go through buildout-versions' finish() method, still.

    option_name=value

  as a short-hand for::

    buildout:option_name=value

Removed '-v' verbosity option from bin/test (just trying something)

Removing the verbosity might make it more obvious where the two failures occur that zc.recipe.testrunner insists exist when running on travis.

favor of the versions sections.

Add default versions to the versions section, if possible.
This means adding a versions section (if necessary/possible).

commit.