commands: Add back missing commands

We accidentally lost the commands
CMD_SCHEDULE_HOSTGROUP_{HOST,SVC}_DOWNTIME when I gave this code a
facelift a while back. This patch adds them back. In theory, it should
be possible to statically ensure that all commands are implemented...

Also fixed a typo in an error message, comamnd -> command.

This fixes #71.

Signed-off-by: Anton Lofgren <alofgren@op5.com>
Merge pull request #72 from catharsis/issue-71

Issue 71