Refactor automatic method selection.

Add an "is_supported()" function to the different methods so that each
method can include whatever logic they wish to indicate if they are
supported on a particular machine. Previously, methods/__init__.py
contained all of the logic for selecting individual methods. Now, it
iterates through a list of possible options and stops on the first
method that it finds that is_supported().

Currently, the decision is made based on the presence of programs in
the PATH. In the future, things such as the platform sshuttle is
running on could be considered.

tests/client/test_firewall.py

@@ -116,6 +116,8 @@ def test_main(mock_get_method, mock_setup_daemon, mock_rewrite_etc_hosts):
     assert mock_setup_daemon.mock_calls == [call()]
     assert mock_get_method.mock_calls == [
         call('not_auto'),
+        call().is_supported(),
+        call().is_supported().__bool__(),
         call().setup_firewall(
             1024, 1026,
             [(AF_INET6, u'2404:6800:4004:80c::33')],