Don't require the remote server to have sshuttle installed.

Instead, grab our source code, send it over the link, and have python eval
it and then start the server.py main() function.

Strangely, there's now *less* horrible stuff in ssh.py, because we no longer
have to munge around with the PATH environment variable.  And this
significantly reduces the setup required to get sshuttle going.

Based on a suggestion from Wayne Scott.

ssnet.py

@@ -1,5 +1,6 @@
 import struct, socket, errno, select
-from helpers import *
+if not globals().get('skip_imports'):
+    from helpers import *
 
 HDR_LEN = 8