Make mvn build accomodate protobuf files

m INSTALL

Add a note about how to do maven build/install

m pom.xml.in

Add dependency on protobuf.

Add protoc plugin that generates proto files under mvn
target directory.  This latter plugin is not in mvn central
so need to add its repository to the pom (I learned about
this plugin here

 https://code.google.com/p/protobuf/issues/detail?id=476)

Updated the source plugin and added a null relativePath
to suppress a silly mvn warning.