Add public API to obtain date from any click event

Currently, only `dayClick` events give away the precise date and time that was clicked.

This changeset allows the date and time to be computed from any DOM event, including the one that is passed from an `eventClick`. This allows the data to be obtained from within an `eventClick` callback, while still maintaining total API compliance.

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This is an update for v1.6 of arshaw#79.