Update and improve Drush integration

The Drush integration does not follow recent Drush best practices - the attached patch makes the following changes:

• Use standardized command callback names.
• Move validation logic to dedicated validate hooks.
• Switched the cache clearing to use the built in Drush cache clear (cc) command (now is "drush cc civicrm").
• Use drush_log in place of drush_print for log messages (basically drush_print is only for "data output" or output directly associated with interactive prompts).
• Use drush_set_error in place of drush_die for errors - passing the command return value back via callback returns and adding error key strings (used to identify and react to errors in automated systems such as Aegir).
• Added aliases for a few commands (feedback welcome on which commands are commonly used, and if the aliases grok).
• Fixed a few inconsistencies, and added TODOs for some potential other things we could look at improving.

I have tested a few of the commands on a local install, but this could probably use some more exhaustive testing before it is committed (I didn't test install/upgrade commands, and I didn't test what happens with older Drush and CiviCRM versions).