Validation fix for strict datechecks

Should we be using GenericValidator for validating dates? GenericValidator uses `SimpleDateFormat` under the hood (http://grepcode.com/file/repo1.maven.org/maven2/commons-validator/commons-validator/1.3.1/org/apache/commons/validator/DateValidator.java), whereas we use JodaTime elsewhere to parse dates.

Any differences in behavior between SimpleDateFormat and JodaTime could cause bugs in maestro. Can we switch this method to use JodaTime instead?