Add read preference config option

This makes [MongoDB read preferences](http://docs.mongodb.org/manual/core/read-preference/) configurable. Usage example:

    JsonObject mongoConfig = new JsonObject();
    mongoConfig.putString("readPreference", "nearest");
    container.deployModule("io.vertx~mod-mongo-persistor~versionnumber", mongoConfig);

Related GitHub issue: https://github.com/vert-x/mod-mongo-persistor/issues/48