Recalculate the interval value

I have trouble with interval. If I try to setup interval after initialization, it still use first interval.
But the string of the rule is right.

```ruby
rule = Rule.daily.until(Date.new(2013,5,1)) # => Daily until May  1, 2013
rule.interval(3)                            # => Every 3 days until May  1, 2013 
schedule = Schedule.new(Date.new(2013,4,1))
schedule.add_recurrence_rule rule # => [Every 3 days until May  1, 2013]

schedule.all_occurrences # => [2013-04-01, 2013-04-02, 2013-04-03, 2013-04-04, 2013-04-05, 2013-04-06, 2013-04-07, 2013-04-08, 2013-04-09, 2013-04-10, 2013-04-11, 2013-04-12, 2013-04-13, 2013-04-14, 2013-04-15, 2013-04-16, 2013-04-17, 2013-04-18, 2013-04-19, 2013-04-20, 2013-04-21, 2013-04-22, 2013-04-23, 2013-04-24, 2013-04-25, 2013-04-26, 2013-04-27, 2013-04-28, 2013-04-29, 2013-04-30, 2013-05-01]
```

Added patch for `DailyInterval`, because I do not understand the logic to use several intervals for one Rule.

Solution without this patch is to use next:

```ruby
rule = Rule.daily.until(Date.new(2013,5,1)) # => Daily until May  1, 2013
rule.replace_validations_for(:interval, [])
rule.interval(3)                            # => Every 3 days until May  1, 2013 
```