Allow client to handle new enum type change

The enum type has apparently changed, from <int> to <string>.
The method `parse_interfaces` and sub-methods were keyed on
enum <int> and would throw the following error::

  ValueError: Wrong argument type: vcs is not a <type 'int'>

This has been updated in the mapping of datatypes, and a small
doctest added.

Also:

I have made the text of the error slightly clearer (in my view), as I was
initially confused as to whether I was provding an int or should be an int.