Purge historical check states cli

```
$ bin/flapjack purge check_history --days 1
Purged 2 historical check states over 1 checks.
```

and specific to a named check:

```
$ bin/flapjack purge check_history --days 0 --check jklfjdksl:567567
Nothing to do

$ bin/flapjack purge check_history --days 0 --check foo-app-01:HTTP
Purged 2 historical check states over 1 checks.
```
This should probably be

```ruby
Flapjack::Data::EntityCheck.for_event_id(@options[:check], :redis => redis, :create_entity => true)
```

as the `EntityCheck.all` will return an array of objects as well, and then you could fold the two maps below into a single one.