DocBlocks for services don't use absolute namespaces

[SimantovYousoufov]

Preface
I have not added any tests for this PR because I was unsure how to actually run the test suite. If someone can provide some insight into how to run the suite, I'll add them in whatever tests are required ASAP.

Issue
These examples use code from the gRPC example.

Currently, the generated docblocks for services return:

    /**
     * @param routeguide\Point $input
     */
    public function GetFeature(\routeguide\Point $argument, $metadata = array(), $options = array()) {
      return $this->_simpleRequest('/routeguide.RouteGuide/GetFeature', $argument, '\routeguide\Feature::deserialize', $metadata, $options);
    }

These doc blocks are

1. Not using absolute namespacing, breaking the typehints in IDEs.
2. Not accurately documenting input params and their types ($metadata and $options).

Proposal
Add logic to DrSlump\Protobuf\Compiler\PhpGenerator@compileStub to handle the above cases based on the RPC type.

It will now return:

    /**
     * @param \routeguide\Point $argument
     * @return \routeguide\Feature
     */
    public function GetFeature(\routeguide\Point $argument, $metadata = array(), $options = array()) {
      return $this->_simpleRequest('/routeguide.RouteGuide/GetFeature', $argument, '\routeguide\Feature::deserialize', $metadata, $options);
    }
    /**
     * @param \routeguide\Rectangle $argument
     * @param array $metadata
     * @param array $options
     * @return \Grpc\ServerStreamingCall
     */
    public function ListFeatures($argument, $metadata = array(), $options = array()) {
      return $this->_serverStreamRequest('/routeguide.RouteGuide/ListFeatures', $argument, '\routeguide\Feature::deserialize', $metadata, $options);
    }
    /**
     * @param array $metadata
     * @param array $options
     * @return \Grpc\ClientStreamingCall
     */
    public function RecordRoute($metadata = array(), $options = array()) {
      return $this->_clientStreamRequest('/routeguide.RouteGuide/RecordRoute', '\routeguide\RouteSummary::deserialize', $metadata, $options);
    }
    /**
     * @param array $metadata
     * @param array $options
     * @return \Grpc\BidiStreamingCall
     */
    public function RouteChat($metadata = array(), $options = array()) {
      return $this->_bidiRequest('/routeguide.RouteGuide/RouteChat', '\routeguide\RouteNote::deserialize', $metadata, $options);
    }

I think the logic for how docblocks and actual method code are generated can be rewritten to generate them in parallel and merge them into $s after generation, but I'm hesitant to make these changes without knowing how to use the test suite and the maintainer(s) desires.