Added support for entering namespaces using absolute paths

[cnelson]

Allows for specifying a full path to a file which represents a namespace rather than a pid.

This is intended to support entering namespaces created with other tools like ip netns add

The Interface is unchanged, you specify a path instead of a numeric pid as the first positional param to the context manager:

with Namespace('/var/run/netns/foo', 'net'):
    pass

Fixes #5

[hjacobs]

👍 there might be better/smarter/cleaner solutions (I guess), but I think that's good enough and solves the problem 😄

[cnelson]

At some point it'd probably be a good idea to add the CLONE_NEW* constants and stop passing 0 as the nstype when calling setns() so we are absolutely sure they are entering the namespace type they requested.