Add joda-time support to BasicJavaGenerator

* Add joda-time as a dependency in pom.xml
* Add import mappings for commonly used joda-time classes

Not sure if it's controversial to have joda-time as a dependency by default, as it's not needed unless you happen to use `DateTime` in your model classes, but it was useful in my case.

I think this would be convenient for a lot of people, and the worst case scenario is you end up with an unneeded dependency on joda-time in your pom.