Model generation: add support for java.util.Calendar

[ldaroczi]

Our JPA entities use the java.util.Calendar type for persisting time stamps.
Currently the workaround is to annotate the fields with:
@ModelField(type = ModelType.DATE, dateFormat = "time")

[ralscha]

Thanks for the report. Added a fix that recognizes Calendar as a date type.
You can test it with the latest 1.2.3-SNAPSHOT release.