[proxy] Add tls support

Caveats:
* TLS files must be present on the target server (and readable)
* TLS file paths cannot contain spaces
* TLS file paths must be absolute (i.e. cannot be relative)
* Bash argument parsing is terrible

.gitignore

@@ -47,3 +47,4 @@ prog/weaveexec/weavewait
 prog/weaveexec/sigproxy
 prog/weaveexec/docker*.tgz
 Vagrantfile.local
+test/tls/*.pem