Implementation du pattern strategy pour print les reponses
permet de choisir entre réponse JSON et BSON
	new file:   io/BsonEcho.java
	new file:   io/Echo.java
	new file:   io/JsonEcho.java
	modified:   model/DataBean.java
	modified:   model/Transaction.java
	modified:   servlets/LoginServlet.java
	modified:   servlets/LogoutServlet.java
	modified:   servlets/TransactionServlet.java
	modified:   servlets/UserServlet.java