Enabled sort and filter options in the flapjack UI.

First of all, I am not much of a UI person, so feedback is very welcomed. This change enables both sort and filtering functionality on the "All Entities", "Failing Entities", "All Checks" and "Failed Checks" tables in the flapjack UI. It was created using the tablesorter library, because there is support for a filter plugin called "widget-filter".

See http://tablesorter.com/docs and http://mottie.github.io/tablesorter/docs/example-widget-filter.html.

For sorting on the "Last State Change"/"Last Update"/"Last Notification" columns, a custom parser was added according to http://tablesorter.com/docs/example-parsers.html.

I had to drop the rowspan of the entity column for the check pages, because filtering with a rowspan is nearly impossible, or would require a significant change in how the UI presents the event data.

These changes were tested on the latest versions of Chrome, Firefox and Safari.