Added logic for generic search functionality

.../it/inaf/ia2/gms/controller/GroupsTabResponseBuilder.java

...ain/java/it/inaf/ia2/gms/controller/SearchController.java

...va/it/inaf/ia2/gms/model/response/SearchResponseItem.java

...va/it/inaf/ia2/gms/model/response/SearchResponseType.java

...va/it/inaf/ia2/gms/model/response/UserSearchResponse.java

gms/src/main/java/it/inaf/ia2/gms/persistence/GroupsDAO.java

gms/src/main/java/it/inaf/ia2/gms/service/GroupsService.java

.../main/java/it/inaf/ia2/gms/service/GroupsTreeBuilder.java

gms/src/main/java/it/inaf/ia2/gms/service/SearchService.java

...java/it/inaf/ia2/gms/controller/SearchControllerTest.java

.../test/java/it/inaf/ia2/gms/service/SearchServiceTest.java