Remove old Navigator class, now Proxy is done

Navigator has now been replaced with Proxy, which does much the same
thing, but without the dependency on the restnavigator module. The main
improvements in Proxy over Navigator are:

1. Proxy does not require the presence of the "cbase" and "sbase" CURIEs
to identify collections and scalars (a.k.a. resources). Instead, it
looks at the links dictionary in a HAL resource and determines whether a
link is to a resource or collection based on whether that link is a
single dictionary or an array of dictionaries, respectively. Proxy will
accept whatever CURIEs the server decides to sent (or none at all).

2. Attempts to dereference a link return None if the resource does not
exist on the server (HTTP 404 response), or a Proxy object for the
resource if it does (HTTP 202 response). And when you have that Proxy
object, you have a local copy of all its properties, so you can safely
access them without fear of running into exceptions. This is a change
from Navigator's behaviour of *always* returning a Proxy for a link and
only fetching when you tried to do an attribute access, so in the case
of a non-existant resource you could check that the link was not None,
and then unwittingly find yourself with an exception when trying to
access an attribute.

3. Proxy is completely independent of the restnavigator; it directly
uses the requests module to talk to the server