Allow TimeSlice to work on time offsets (relative time)

[hovren]

TimeSlice requires start and end to be absolute timestamps within the event stream.
In many cases we might be interested to get "the first five seconds of data" which is currently a bit cumbersome.

I suggest adding a relative=True|False keyword to TimeSlice, defaulting to False to retain the current mode. When relative=False the start and end timepoints are to be considered offsets to the start of the stream, instead of absolute timestamps.