[rollup reactor] allow for scaling rollups by passing in a function that accepts the period as an argument
[test] add test case for a function used as interval for rollup
[api test] Properly pass along the expended duration in which rolled up events have been received in addition to the periods.
[minor fix] remove unnecessary require
Merge pull request #52 from nodejitsu/scaling-rollup

[rollup reactor] allow for scaling rollups by passing in a function for the interval