MultiMap implicit conversion

Added implicit conversion for org.vertx.java.core.MultiMap to scalas builtin mutable.MultiMap. The implicit conversion method is added to http package object.

Changed signatures for HttpServerRequest.{headers, params} and HttpServerResponse {headers, trailers} from mutable.MultiMap[Any, Any] to mutable.MultiMap[String, String] since internally a key-pair of string values is stored. The implicit conversion creates a new MultiMap instance, hence any mutation done to that instance will not affect the internal representation. Say you want to add additional headers/trailer to HttpServerResponse, you should use put{Header, Trailer} methods provided rather than adding them manually to the MultiMap instance acquired via headers/trailers methods.