Updated README to use newer version (2.3.0) of protobuf-java.

Also added notes on stale files check.
[maven-release-plugin] prepare release maven-protoc-plugin-0.1.9
[maven-release-plugin] prepare for next development iteration
Clean up variable declarations and fixed casing of protoFiles variable.
GH-8 Fixed bug where the Java compiler was not being invoked correctly.

The bug occurs if the class files are out of date,
but the protoc generated source is ok.
[maven-release-plugin] prepare release maven-protoc-plugin-0.1.10
[maven-release-plugin] prepare for next development iteration
Change file extension to .proto
Added ability to include protoc plugins as part of build