Fix an obvious bug in the add_isoextras exception.

Signed-off-by: Chris Lalancette <clalancette@gmail.com>
Fix up some documentation.

Signed-off-by: Chris Lalancette <clalancette@gmail.com>
Rewrite do_install to be more readable.

Signed-off-by: Chris Lalancette <clalancette@gmail.com>