UI for selecting custom captured fields

When a user clicks on the 'Show Differences' button for multiple executions, they can select via a drop-down menu, which custom captured fields (date, ip, action, etc) are used in the comparison of events in different executions. That is, in addition to the event string, the user can specify what other fields they want to compare the nodes by.

[Issue created by pattypcw: 2015-03-05]