Refactor computers class
Refactor computers class

- Added default query attributes property
- Requesting info on a computer name that is null will throw an
exception
- Requesting info on a computer when LDAP is not bound will throw an
exception
- Remove needles validation since other methods perform this already

jal617b/Adldap2,kichetof/Adldap2,ciadch/Adldap2,dlundgren/Adldap2,Adldap2/Adldap2