Added support for java.sql.Timestamp

While using this great plugin just realized that it was throwing a GroovyCastException while building a domain object that contained fields of type java.sql.Timestamp.
```
org.codehaus.groovy.runtime.typehandling.GroovyCastException: Cannot cast object 'Fri Oct 03 15:28:24 COT 2014' with class 'java.util.Date' to class 'java.sql.Timestamp'
	at grails.buildtestdata.handler.NullableConstraintHandler.handle(NullableConstraintHandler.groovy:25)
	at grails.buildtestdata.DomainInstanceBuilder.createMissingProperty(DomainInstanceBuilder.groovy:198)
	at grails.buildtestdata.DomainInstanceBuilder.populateInstance(DomainInstanceBuilder.groovy:158)
	at grails.buildtestdata.DomainInstanceBuilder.build(DomainInstanceBuilder.groovy:136)
	at grails.buildtestdata.DomainInstanceBuilder.build(DomainInstanceBuilder.groovy:135)
```
Added a straigthforward modification so this datatype is supported.