Don't return too early from destroy action

`return` in the block return from the whole method. So the machine id
was not reset (and provision sentinel not removed), and next (possible)
middleware action was not called.

Also fix the indentation. =)
Add ProvisionerCleanup to the destroy action middleware stack

Call cleanup method of the provisioners (Vagrant >= 1.3.0).
This is used at least by chef-client provisioner to optionally
delete the client and node from the Chef server.
Add confirmation for `vagrant destroy`

The standard behavior is to ask confirmation from the user unless
`--force/-f` option is specified.
Merge pull request #79 from tmatilai/destroy-return

Don't return too early from destroy action
Merge pull request #80 from tmatilai/provisioner-cleanup

Add ProvisionerCleanup to the destroy action middleware stack
Merge pull request #82 from tmatilai/destroy-confirmation

Add confirmation for `vagrant destroy`
Add changelog and bump version for release.