Creat sprint registracion form

Whitout saving sprint in database
Merge branch 'sprint_interface'

Conflicts:
	src/si/fri/tpo/gwt/client/form/registration/AbstractSprintRegistrationForm.java
	web/dscrum/dscrum.nocache.js
Fixed bug with user search.

The bug was nullPointerException if user doesn't exist. Now a message is displayed instead of crashing and erroring.