Add autostart as an option for vagrant boxes

Added specs for the environment and vagrantfile. Added a methd to retrieve
autostart_machine_names. Changed the plugin up/command to use autostart_machine_names
when no argument was given to the command.
Merge branch 'feature/autostart' of github.com:ramondelafuente/vagrant into ramondelafuente-feature/autostart
Merge branch 'ramondelafuente-feature/autostart' into vagrant-next
Update CHANGELOG