Run avocado when called by 'python -m avocado'

Add a main entry point when avocado is called
by the command line option 'python -m avocado'.

Signed-off-by: Rudá Moura <rmoura@redhat.com>
Merge pull request #292 from ruda/main_entry_point

 Run avocado when called by 'python -m avocado'