Add cron definition english translation

This pull request add an attribute `translation` to each entry which holds an english translation to the cron definition.

It makes use of the gem [cron2english](https://github.com/pjungwir/cron2english).

**Example**

```ruby
schedule = Crontab::Schedule.new('@hourly')
command = 'echo hello'
cron_definition = '0,30 0 * * *'
entry = Crontab::Entry.new(schedule, command, cron_definition)
entry.translation
 => ['0 and 30 minutes past', 'midnight of', 'every day']
```