and introducing list_dependencies_command.  (Ted, Holger)

session.Reporter: delete _starttime at exit

Reading the tox.ini config file and processing it is done as one task, yet
processing the minversion is done much later, as such if there is a bug fix
in a newer version or a change that makes the tox.ini file incompatable with
older versions tox will bail with a stack trace instead of a
"Your tox is too old" message.

This moves the version check as early as it can possibly be done, the very
first value read out of tox.ini is minversion and it is immedeiatly compared
to tox.__version__.

The __enter__ of with statement extend the action object
with _starttime and assert if it had been already extended.
Therefore the __exit__ should remove the extra attribute
of the action.