New gameplay design added & bug corrections

                                               * Now check if you have a specified amount of resources to join a team.

                                               * You can costumize these resources using config.yml

                                               * Corrected a bug where joining a full team leads you to have a 'null' team (NullPointerException !)

                                               * Adding principle of Cost and equivalence between a material and a number of materials.

                                               * Correcting serialVersionUID

                                               * To use the new config.yml, delete the old one