Implement a BRB command

[izzy]

There should be a plugin that allows to set a secondary space status to "be right back".

Those updates SHOULD then be distributed to the space status JSON. That way the status page, the Twitter status bot and the IRC topic plugin can react to this change and set their messages appropriately.

I propose to set the command to "!brb". This MUST NOT be set by anyone outside the space.

As the knowledge of which user is inside the space is a prerequisite issue #11 MUST be implemented first.

Before any implementation can take place there MUST be a way to set and retrieve a secondary status from the status JSON as well.