Added HTTP proxy support

Tested with `173.234.237.198:3128` HTTP proxy and my TCP bootstrap node `192.210.149.121:3389` , `F404ABAA1C99A9D37D61AB54898F56793E1DEF8BD46B1038B9D822E8460FAB67`.
The node should be up, proxy might not.

Apparently it's not that easy to find HTTP proxies that support CONNECT. I had to go over 20 HTTP proxies, getting from no response (timeout), immediate disconnect or some HTML-formatted errors.

Even working HTTP proxies might not connect to a working TCP bootstrap node from the first time, you have to keep trying connecting. Tox doesn't retry to establish connections, so this should be done by clients by bootstraping with some delay until tox is connected.
We could also do
```C
typedef enum {
    TOX_PROXY_NONE,
    TOX_PROXY_HTTP,
    TOX_PROXY_SOCKS
} TOX_PROXY_TYPE;
```

and get rid of 
```C
    uint8_t proxy_enabled;
```

Not sure how do I feel about this, but that's an option.