Tests subproject added.

* optional testing subproject added
* get_handler_t has now deferred dependency injection
* get_handler_t json parsing is now handled by rapidjson library
* request_handler_t on call now operates with deferred reference instead of copy
* adequate error handling and normal usage has been tested
Refactoring and test coverage.

* index methods have been tested
Elasticsearch Service: code refactoring.
Elasticsearch Service: disable testing and delete support.
Merge branch 'master' of github.com:cocaine/cocaine-plugins