AutoCommand fixes.

Don't use a sorted map, as the commands aren't sortable.
Unsubscribe when stopping, instead of subscribing a second time...

Change-Id: Ib0301c9089de950dc4341c44169d94cb8dc2b2b4
Reviewed-on: http://gerrit.dmdirc.com/3915
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager