Minor Drush improvements

Figured I would submit some quick edits as I reviewed this nice project. Sorry - didn't test them.

hook_drush_help() is rarely used now. Its just helpful when you want the detailed help for your command to have a longer description than what appears in the listing (i.e. in the command definition).

drush_log() puts messages into the log which is handed back properly when making remote calls. So its preferable if you ever start doing drush @remote site-make. Also, drush_print() puts stuff on stdout and I think these status messages are better suited to stderr.

Also, DRUSH_COMMAND is better as it works if someone's drush is not on their $PATH. Can happen when you use a different version (likely, as this requires Drush master).