Implement php_package optional parameter

Protocol Buffers support cross-language proto files by using optional parameters (see [Defining Your Protocol Format](https://developers.google.com/protocol-buffers/docs/javatutorial)).

This commit implements an optional parameter - `php_package` - that can be used to specify a php specific package name / namespace. For example:

    package MyProject;

    option java_package="com.foo.bar";
    option php_package="app.messages.foo.bar";

This proto file can now work both Java and PHP applications. I've updated `README.md` with an example.