Updated the location for `protobuf-maven-plugin`

Uses Google's C++ library for .proto parsing and uPB for protobuf
encoding/decoding; it supports both proto2 and proto3 syntax and
encoding/decoding is more than 10 times faster than pure-Perl
Google::ProtocolBuffers.

Add reference to Protocol Buffers Dynamic Schema (https://github.com/os72/protobuf-dynamic)

1.only support proto3
2.support oneof map
3.support nested message and enum for as3