Added a simple TeacherClientService.

It demonstrates how I most likely want to interact with the REST server. It requires a bit more work to parse the JSON responses, but I find it worth it when trying the different frameworks which could do it for me.
Added possibility to create a teacher.

 Also added simple validation framework for validating forms and preventing complete actions for them until the form is valid.
Added SamePasswordValidator.

Also added focus on first field that fails validation.