* Uses build-helper-maven-plugin to add generated sources to the classpath
* Fixes an issue building with newer versions of the maven-compiler-plugin
  (See https://issues.apache.org/jira/browse/MCOMPILER-240)

gson 2.3 has internal bug that it doesn't work with some versions of maven.

gson 2.3 has internal bug that it doesn't work with some versions of maven.

The default behaviour of throwing an exception remains

This includes all internal changes from around May 20 to now.

- Using custom URL for types in Any will no longer throw an
InvalidProtocolBufferException in JsonFormat
- Fixes #1128

Java files are moved to un-do the hack in the prevous commit, which
moved the java files to the original position for integration.

Moving the files to their original location, so that opensource changes
can be picked during the internal merge. Those files will be moved into
the correct location after merging with internal code.

Note: do NOT merge this into master without the other internal
down-integration commit.

protobuf/java will become a parent pom that will contain two modules:

core - contains all of the code for the protobuf-java artifact
util - contains all of the code for the protobuf-java-util artifact

Also cleaned up various Maven warnings.

Change-Id: Ibb3fe6f919cc7ca0df91da5e1697ba33d259e433

Change-Id: I3daf32cb93dfe0fca38f8b48615ccfc307f748e9

Protoc, C++ runtime and Java runtime are updated to v3.0.0-beta-1, other
languages are updated to v3.0.0-alpha-4.