feat(): allow function to calculate the delay for each iteration.

Request Type: feature

Component(s): misc core

Impact: small

Complexity: small

Detailed Description:

Example use case:

    var START_DELAY = 1000;

    $interval(
      function() { ... },
      function(iteration) { return ((Math.pow(2, iteration) - 1) / 2) * START_DELAY; }
    );