Add support to HTTP proxy, fix issue #22

Add support to HTTP proxy, fix issue #22

Sure, I totally agree that standard HTTP/1.1 requests are better (they are essential when going through a proxy)

With proxy:

```
GET http://translate.google.com/translate_a/t?client=tr HTTP/1.1
Host: translate.google.com
Connection: close
```

Without proxy:

```
GET /translate_a/t?client=tr HTTP/1.1
Host: translate.google.com
Connection: close
```

Since `Host` is now included in our request, we could just drop `HttpPathPrefix` and use the latter:

```
url = "/translate_a/t?client=t"              \
```

(It works in my case. Better if you could test it with your proxy)