Merge pull request #4 from madAndroid/master

Avoid trying to list snapshots on machines that have not yet been created - this conditional isn't necessary in the other commands because that's taken care of by the actions.
Clarify messaging
Bump version for release
Check if VM is already powered off before halting

Before this commit, rolling back a snapshot from a VM in a poweroff
state results in the below error:

Stderr: VBoxManage: error: Invalid machine state: PoweredOff (must be
Running, Paused or Stuck)

This commit adds a check to skip the halting step if the machine is
already off.
Replace execute call with vagrant read_state function

This is per a comment from jtopper. Using the provider's read_state
function is much cleaner than calling out to VBoxManage, since we
already have access to the state data.
Merge pull request #5 from supercow/avoid_halting_when_halted

Check if VM is already powered off before halting
Bumped version for release
Expand skip-halt logic to include aborted state

We already don't bother trying to halt a machine in the poweroff state.
This commit goes further and says that if a machine is aborted, we
aren't going to try to halt it. This is necessary to successfully do a
`vagrant snap rollback` against an aborted machine.
Merge pull request #6 from reidmv/avoid_halting_when_aborted

Expand skip-halt logic to include aborted state
Bumped version for release
Added contributors list to README.md
Deal with snapshots more sensibly.

Either VBox changed how they list snapshots, or I got this wrong
the first time, but now we pick up all the snapshots and choose
the most recent.

The VMWare plugin also chose the first, rather than the last snap
whic was incorrect.

Closes #7.
Bump version for release.
merge upstream changes