Added implicit conversion for org.vertx.java.core.MultiMap to scalas builtin mutable.MultiMap.

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.
 merge raniejade-multimap-implicit