Support removing Event Listener & avoid initializing multiple times

[alfonsrv]

Describe the bug
When triggering multiple initializations on the same element, NJTimePicker won't check whether or not an instance is already associated and will simply create a new one. This can cause one element to spawn multiple NJTimePickers overlaying each other when clicking the input field.

Expected behaviour
When NJTimePicker is already initialized for an element, it should not be initialized again. Similarly an easy method of removing a NJTimePicker like NJTimePicker.remove("#timepicker") would be nice-to-have.