Merging buildout-versions' functionality into buildout

As discussed on the mailinglist, I copied buildout-versions' functionality into buildout itself. 
The pull request includes tests and documentation.

From the changelog:

- Integrated the buildout-versions extension into buildout
  itself. For this, a few options were added to buildout:

  - If `show-picked-versions` is set to true, all picked versions are
    printed at the end of the buildout run. This saves you from running
    buildout in verbose mode and extracting the picked versions from the
    output.

  - If `update-versions-file` is set to a filename (relative to the buildout
    directory), the `show-picked-versions` output is appended to that file.

- Also from buildout-versions: if a `python` version is given in the
  versions, buildout now checks if it is running with that python
  version. Handy if you know if the buildout only functions with a specific
  python version.