Changing image commands to be async functions

Instead of having separate classes for each async command, they now are
functions inside StandbyMode which are run in separate threads. I also
cleaned up the function signatures for StandbyMode.
Fixing quoting
Making async and sync calls consistent
Added an async_command decorator

Functions with the async_command decorator will be run inside an
AsyncCommandResult. The tests are passing but aren't really unit tests,
as they always run inside the async decorator.
Fixing single vs double quotes
Use thread methods rather than time.sleep to wait
Merge pull request #49 from rackerlabs/async-standby

Async standby