KNIFE-260: Added start / stop commands.

Add knife ec2 commands for starting / stopping the servers. Following commands should be added:

knife ec2 server start -i instance-id,instance-id -t tag=pattern*,tag=value
knife ec2 server stop -i instance-id,instance-id -t tag=pattern*,tag=value

Options:

-i, --ids: list of instance ids
-t, --tags: list of tag names and values or patterns