Add new Proxy class

The Proxy class is the new implementation of the HAL navigator, to
replace the existing one based on the restnavigator module. Currently it
does not support all functionality, so it is going to exist for the time
being alongside hyperdns.hal.navigator.Navigator, so we can gradually
transition to it, with the goal being to avoid API changes unless
absolutely necessary.

The test_proxy00_properties.py tests are identical to those used for the
current Navigator object, but use the new Proxy class instead.