add local-date & today functions that use org.joda.time.LocalDate

This is useful for modeling entire days separately from time and timezones.
add local-date example to README
add to-date-time coerce for LocalDate
add to-local-date coerce to convert date types to LocalDate
add format parse-local-date and unparse-local-date
add default-time-zone to to-local-date so that it is localized to the
current time zone
Merge pull request #58 from cap10morgan/add-local-date

Add LocalDate support