Add support for full displayed day and month names

Added support for:
- full displayed day names in the daypicker header instead of abbreviated ones
(`full_day_names_in_header` option)
- full displayed month names in the month picker instead of abbreviated ones (`full_month_names_in_month_picker` option)

With this small change you can now create nice calendar-style datepickers out-of-box with Zebra_Datepicker with no unneccessary abbreviations:
![bigcal2](https://f.cloud.github.com/assets/304384/2239032/1cefe1da-9c10-11e3-8502-285da44c4ace.png)
![bigcal1](https://f.cloud.github.com/assets/304384/2239031/1cefe734-9c10-11e3-8668-bcc990349cb2.png)

If needed, I can provide a sample big calendar skin and create an HTML in the examples folder with it based on the existing examples.