Organize the options better

[zhangkun83]

Most of the options are exposed as top-level conventions, which pollutes the project name space and doesn't read well. We'd like them to be better organized, possibly put them under a big top-level protobuf {} block and use a more Gradle-ly style, e.g., change

protocDep = 'com.google.protobuf:protoc:3.0.0-alpha-2'
protobufNativeCodeGenPluginDeps = ["grpc:io.grpc:protoc-gen-grpc-java:0.1.0-SNAPSHOT"]

to

protobuf {
  protocExecutable {
    artifact 'com.google.protobuf:protoc:3.0.0-alpha-2'
  }
  codeGenPlugins {
    grpc {
      artifact 'io.grpc:protoc-gen-grpc-java:0.1.0-SNAPSHOT'
    }
  }
}

[zhangkun83]

0.5.0 has been released.