Add support for .equals() to Java structs.

Add support for .equals() to Java structs.

- Add a new method to t_java_generator that generates a .equals() method.
- Add a correct but useless implementation of hashCode for structs.
  (This is required by conventino when overriding .equals().)
- Add java_package to DebugProtoTest and OptionalRequiredTest.
- Add a new structure to OptionalRequiredTest to assist testing the behavior
  of .equals() with respect to null and unset fields.
- Clean up test/java/build.xml a bit.  (It still has a ways to go.)
- Add EqualityTest.java to test .equals().
- Add IdentityTest.java to test that writing and reading a structure
  preserves equality.

Tested by runnint atn test.
I also looked at the generated code for OptionalRequiredTest.thrift.

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@665541 13f79535-47bb-0310-9956-ffa450edef68