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/server.h

@@ -52,10 +52,13 @@ typedef struct server_ctx {
 typedef struct server {
     int fd;
     int stage;
+    int auth;
+
+    obfs_t *obfs;
+
     buffer_t *buf;
     buffer_t *header_buf;
 
-    int auth;
     struct chunk *chunk;
 
     struct enc_ctx *e_ctx;