Add HTTP/TLS obfuscating. [SIP001] (#1009)

Add experimental HTTP/TLS obfuscating as an **optional extension** of shadowsocks protocol.

More discussions can be found here: https://github.com/shadowsocks/shadowsocks-org/issues/26

As this feature is still a SIP (Shadowsocks Improvement Proposal), it's very unstable and experimental. So, 

1. Don't enable it unless you know what it is.
2. Be very careful when using it in production environment.

src/utils.c

@@ -358,6 +358,12 @@ usage()
     printf(
         "       [--firewall]               Setup firewall rules for auto blocking.\n");
 #endif
+#endif
+    printf(
+        "       [--obfs <http|tls>]        Enable obfuscating: HTTP or TLS (Experimental).\n");
+#ifndef MODULE_REMOTE
+    printf(
+        "       [--obfs-host <host_name>]  Hostname for obfuscating (Experimental).\n");
 #endif
     printf("\n");
     printf(