Add check requests

This adds the ability to manually run check requests via the dashboard. Will only work with Sensu version '0.11.0' as the API appears to have changed in 0.12.0 and I can't find any documentation on it (assume WIP as the default version Apt installs is 0.11.0).

Apologies for the code, learning Ruby & Coffee on the fly here - will not be offended if this request is deleted/not-merged!