Changed some of the checks to support strings as vlan identifiers as
supported by RFC 2868.

This change does not claim to be exhaustive. We probably have other
checks like this where we assume the identifier is an integer instead
of a string.
Corrected incorrect comment.
Upcased vlan to VLAN.
Merge pull request #90 from louismunro/vlan_by_name

Changed some of the checks to support strings as vlan identifiers as