add support for deleting snapshots #14
VBoxManage snapshot delete doesn't take --pause argument
add snapshot not deleted messages
add snapshot not deleted messages - and actions to control deletion based on HasSnapshot
ensure snapshot name for deletion is mandatory

... and for rollback, check if snapshot exists, if name given
pass snapshot name to has_snapshot?
don't pass default timestamp named snapshot to deleteSnapsot
match on string, not regex
keep logic out of 'command', move to 'action'
be more explicit in ui ouput
better has_snapshot? handling of no snap_name being passed
add argument to has_snapshot? - to query whether particular snap shot exists
improve output during snapshot operations
address comment on PR - raise MissingArgument
add result checking for snapshot delete
better output handling for named snapshots, and revert changes to non-named snapshots
handle unamed snapshot correctly for rollback
remove :prefix => false - now allows us to see which machine we're operating on
handle empty snapshot list correctly for has_snapshot?
add default for has_snapshot? method - otherwise vagrant-cucumber fails