Add support to HTTP proxy, fix issue #22

Add support to HTTP proxy, fix issue #22
By the way, I'm sure this (using full Request-URI) is gonna work as before, with or without proxy:

```
url = HttpProtocol HttpHost "/translate_a/t?client=t"       \
```

Then we are able to avoid `HttpPathPrefix` and have **the same request** for both cases. What do you think?