_tunnel_host compatibility for Python 2.6.(0-2)

The _tunnel_host attribute was added in Python 2.6.3, so older Python 2.6's
failed when urllib3 would attempt to access the _tunnel_host attribute. This
change checks for the attribute's existence before accessing it. This fixes
Github issue #306.

Adds tests for this change.
python: how does it work
Merge pull request #307 from kevinburke/tunnel-no-tunnel

_tunnel_host compatibility for Python 2.6.(0-2)