Don't allow zero/negative INTERVAL, assume 1

Right now its possible to add an RRULE with INTERVAL=0 or INTERVAL=-1. This will go through and as soon as we start iterating occurrences we will either throw an exception ("protecting you from death by recursion") with INTERVAL=0 and I haven't checked what happens with INTERVAL=-1 (likely death by recursion).

This patch assumes 1 when reading in a such RRULE. I haven't gone so far to check this.rule.interval each time, I think this check should be enough.

Complete with test and browser build, I hope I didn't miss anything this time.