Add 'ignore_failure' option to the rightscale.server_array.Execute actor (issue #81)

This patch adds a required parameter 'ignore_failure' to the Execute script actor. Setting it to False ensures the current behavior, where the result of the script (success/failure) matter. Setting it to True means that the actor will return True regardless of the script execution result.

Note: I think we should hold off on merging this until we handle issue #60. If we merge this now, it will break existing scripts for people because they will have to add this option to the JSON. If #60 is not addressed in v0.1.1, then this should be held until v0.2.0.