feat: add namespace property to context.Context

[chrisrossi]

It is now possible to set the namespace on the context, overriding the
namespace set on the client. This makes it easier to write multi-homed
applications which might access different namespaces depending on which
user is logged in or other variable only known at request time.

client.Client.context has a new argument, namespace, which can be
used to set the namespace at request time. Here is an example of you
might do this in a WSGI middleware:

def ndb_wsgi_middleware(wsgi_app):
    client = ndb.Client()

    def middleware(environ, start_response):
        global_cache = ndb.RedisCache.from_environment()
        namespace = get_namespace_from_request(environ)
        with client.context(global_cache=global_cache, namespace=namespace):
            return wsgi_app(environ, start_response)

    return middleware

app = flask.Flask("NDB Example")
app.wsgi_app = ndb_wsgi_middleware(app.wsgi_app)  # Wrap the app in middleware.

Closes #385.

[cguardia]

Looks good to me. Thanks!