Conversion window support

[tomsheep]

Specify how much time a user has to complete the funnel from the moment they enter it.

[joshwalters]

Time in terms of number of steps (they have to continue on in the funnel within X steps), or time in terms of clock time (within 5 minutes of starting the funnel)?

For the clock time solution the timestamp_column would have to be restricted to a specific type (right now its a PRIMITIVE).