Allow enabling of disabled dates. closes #248

Enabling a date will now add it to the disable array with a fourth value `inverted` in the array:

```json
[ 1, 3, [2013, 4, 6], [2013, 10, 4, 'inverted'] ]
```

So this setup will show all mondays as disabled but the 4th of November is enabled.

Adjusted the tests as the expected output for enabling changed.

To test with a visible calendar, open the api page and execute the code in the issue #248.