Enable message handlers being attached to clients

This adds addHandlers which manages a map of message handlers. When
a message is received all handlers that are registered for the message's
type are called.
Add message sending capabilities for clients

Messages are queue by Client::send() and then actually sent on each
invocation of Client::exec().
Added utility method to Server to send a message to all clients
Ignore build/ directory
Move the message handlers up to the Server

 * Client::exec now returns all new messages for the client
 * Message handlers are now held by the Server
 * Calling message handlers is now done by the Server
 * The handler signature has been changed to include the Server
   as well as the Client and JSON entity
Reimplement sending of map hashes

Currently this sits in acceptConnections(), which it probably
shouldn't.
Merge pull request #22 from Holiverh/master

Message sending and receiving