Add support for using Optional<DateTime> in dropwizard-jdbi

Add support for Optional<DateTime> in dropwizard-jdbi by handling NULL-values
in JodaDateTimeMapper. This also fixes a NPE-occuring when using the
existing mapper to plain DateTime-object if extracted column is NULL.
Merge pull request #419 from klette/support-optional-datetime-in-jdbi

Add support for using Optional<DateTime> in dropwizard-jdbi