Add the maint subcommand to the chat bot to fix #664

users can use 'start-in' to say things like the maint should start in '1 hour', '1 day' etc.

flapjack-dev: maint entities /cs-s-hbase03.eng.jiveland.com/ testing start-in: 1 minute duration: 10 minutes

Or 'start-at' to give a specific date/hour to start at like '2014-01-01 07:00'

flapjack-dev: maint entities /collins.jcadev/ test9 start-at: 2014-01-01 07:00 duration: 12 hours

Additionally the user can just rely on the defaults and get a 1 hour maint starting now.
A case statement would be good here eg:

```ruby
maint_start = case
when start_in
  Time.now.to_i + ChronicDuration.parse(start_in)
when start_at
  Chronic.parse(start_at).to_i
else
  Time.now.to_i
end
```