Clients can be associated to one or more Clients Groups.
 - Refactoring of the DataProvider classes.
 - SQL queries for managing the relationship between clients and groups.
 - Improve the documentation of some methods.
 - Graphic User interface.

Closes #17.
Closes #18.
Add the remote XMPP address of clients.
 - The XMPP address of the ESI is included in the business logic and the GUI.
 - Now, WSCaller also includes methods to call the RESTful services related to the Client entity.
Closes #20.