ui functionality:
 - create user
 - delete user
 - sorting by fields "first name", "last name", "email", "activity"
 - filtering by activity (all\active\not active)
 - pagination (can choose page size, can switch page)

implemented:
 - httpBackend mock
 - users resource
 - modal to create user
 - alert when user is created or deleted
 - number of pages depends on number of showed users, not all existing users (for filtering)