DRY up the user model slightly

regular? is just the inverse of staff?
Clean up update_visit_record!

update_visit_record! appears to add user_visit record if one
doesn't exist and days_visited is simply a counter for the number
of visit records.
Added a spec for staff? and regular?
Merge pull request #859 from lengarvey/user_refactoring

Light refactoring of the User model