Implementation of services and minor fixes

- Minor fixes on controllers: Spring is already checking for trailing
  slashes, so no need to write @RequestMapping(value = {"", "/"}...
- Minor fixes on entities: add initialization of collections and embedded
  identifiers on field declaration (resolve null pointer exceptions)
- Started creation of forms backing-bean objects to clean entities of some
  code who shouldn't be there (JSR-303 validation, additional fields for
  JSR-303 validation purpose)
- Created services files to prepare trainings in school with modules,
  courses and classes
- Added service content in SchoolService to allow addition of employees
  and students in a school
- Work-around: some collections content are now EAGER instead of LAZY;
  this resolve some transactional problems but they should return at a
  LAZY initialization in the future with the use of AspectJ instead of
  Spring AOP