Split client/server tests

This allows disabling all client tests using a conftest.py file, if for
example #56 gets merged and the server supports more python versions
then the server.

The server side tests are very incomplete.

sshuttle/tests/server/test_server.py

+import sshuttle.server
+
+
+def test__ipmatch():
+    assert sshuttle.server._ipmatch("1.2.3.4") is not None
+    assert sshuttle.server._ipmatch("::1") is not None
+    assert sshuttle.server._ipmatch("42 Example Street, Melbourne") is None