Add feature that shows cron format as friendly string

As suggested by http://reddit.com/u/HereComeTheMinions
http://www.reddit.com/r/programming/comments/2ma95k/visual_opensource_cron_editor/cm2cxpa

Provide friendly text format explaining cron, such as:

```
*/20 * * 13,12,24 8,11 *
# Every twenty seconds on on the 12th, 13th and 24th, only in august and november.
```

+30 Internet Points

The issue does not name files, tests, or an entry point; begin by locating the cron-format parsing and display code in the repository. Use the supplied cron expression and expected sentence as the acceptance example, and confirm the feature explains the schedule in friendly text.

Written by the indexing model from the issue text.