Add value restrictions for some common date formats

[ssnyder]

The value restrictions are awesome - I'm updating my older code to make use of some of them to make my code that much easier to read/parse and improve the Help as well.

I thought a reasonable addition would be to create additional value restrictions for some common date formats (or allow us to specify the formatting in the markup) would make sense to add as well. This would be for Date and DateTime.

[rvesse]

Related to #73, definitely something on the to do list.

My thinking on date time handling would be to do a separate module - maybe airline-datetime-restrictions? - that brings in http://www.joda.org/joda-time/ as a dependency since JDK Date Time support is pretty rough and I would like to avoid adding extra dependencies to the core if possible.

If you want to take a stab at this yourself there is now documentation on everything necessary for defining new restrictions - http://rvesse.github.io/airline/guide/restrictions/custom.html

[ssnyder]

Thanks!