Fix issue #12: support non-standard DN attributes.

Refactor AttributeType as an interface and provide an enum for standard
attributes as well as a concrete class for "unknown" attributes that uses
the OID for the type and name. Leverage type for proper encoding in
LdapNameFormatter since we can now accommodate OIDs in DN formatting.
Merge pull request #13 from vt-middleware/non-standard-dn-attr

Fix issue #12: support non-standard DN attributes.