Occurrences filtering

This PR implements a filtering option in the Hide drop down menu on the Events page which filters events based on their occurrences parameter. Events should be filtered when the number of occurrences of this event firing is equal to or exceeds the check's occurrences parameter. A unit test for the filter function is included.

This is in reference to https://github.com/sensu/uchiwa/issues/231
In our environment, we enforce ```occurrences => 1``` on all checks unless explicitly set otherwise. That this isn't true for everyone was neglected in preparing this PR. Apologies! Updates to check whether the occurrences parameter exists in filtering and updated unit tests to come soon.