modifies #first and #last to accept a count as a second parameter

Greetings!

A small change to your gem; allows `#first` and `#last` to take a second parameter number, so you can see when the next 5 (or last 5) occurrences of a cron will take place.

```
cron_parser = CronParser.new('0,30 * * * *')
=> #<CronParser:0x007fb6abe7d3d0 @source="0,30 * * * *", @time_source=Time>
next_comming_time = cron_parser.next(Time.now)
=> 2014-06-14 21:30:00 -0400
next_comming_time = cron_parser.next(Time.now, 4)
=> [2014-06-14 21:30:00 -0400, 2014-06-14 22:00:00 -0400, 2014-06-14 22:30:00 -0400, 2014-06-14 23:00:00 -0400]
next_comming_time = cron_parser.last(Time.now, 4)
=> [2014-06-14 21:00:00 -0400, 2014-06-14 20:30:00 -0400, 2014-06-14 20:00:00 -0400, 2014-06-14 19:30:00 -0400]
```
added some coverage to verify functionality