Issue 50 - Add documentation for PHP namespace options

[nigelhough]

This pull request directly relates to https://github.com/protocolbuffers/protocolbuffers.github.io/issues/50

This expands the documentation for PHP Code Generation specifically documenting functionality added in the following two pull requests to protocolbuffers/protobuf.

• https://github.com/protocolbuffers/protobuf/pull/3162
• https://github.com/protocolbuffers/protobuf/pull/4609

For testing and validating the documentation the following .proto files was used

MyMessage.proto

syntax = "proto3";

package foo.bar;

option php_namespace = "baz\\qux";

option php_metadata_namespace = "Foo";

message MyMessage {}

and the following compiler command

protoc --php_out=output MyMessage.proto