command-line pass
* profile create does *not* include redundant '--init', inherited from parent app
* --init message more verbose
* include aliases in items scrubbed from the argv passed to the kernel from the frontend, now that unrecognized aliases throw a warning
* remove plain/pure as aliases, leaving only bool flags
* make <engine|controller>_launcher_class help message more verbose, including example values
* engines inherit work_dir from launching ipcluster by default, rather than using profile_dir
* unimportable launcher classes won't cause a crash in ipcluster
