POC debug information

the log entries in symfony log for example show something like this when a request failed:

```
[2014-07-16 14:27:37] app.CRITICAL: 0 error response "Client error response [status code] 401 
[reason phrase] Unauthorized [url] http://127.0.0.1/" from caching proxy at 127.0.0.1 {"exception":"
[object] (FOS\\HttpCache\\Exception\\ProxyResponseException: 0 error response \"Client error 
response\n[status code] 401\n[reason phrase] Unauthorized\n[url] http://127.0.0.1/\" from caching 
proxy at 127.0.0.1 at /vagrant/vendor/friendsofsymfony/http-cache/src/Exception
/ProxyResponseException.php:29, Guzzle\\Http\\Exception\\ClientErrorResponseException: Client error 
response\n[status code] 401\n[reason phrase] Unauthorized\n[url] http://127.0.0.1/ at /vagrant/vendor
/guzzle/guzzle/src/Guzzle/Http/Exception/BadResponseException.php:43)"} []
```

this is not exactly helpful to figure out what was going on. this POC adds information what was actually sent to the server:
```
BAN / HTTP/1.1 Host: migrosapi.lo X-Host: .* X-Url: .* X-Content-Type: .* X-Cache-Tags: (my-tag)
(,.+)?$ User-Agent: Guzzle/3.8.1 curl/7.19.7 PHP/5.5.12 Content-Length: 0 
```

maybe [twitter](https://twitter.com/dbu/status/489385378252869633) will find a good suggestion how to format this as curl command line. otherwise we can attempt something ourselves or just format this a bit more nicely.
I see an issue here: this makes the exception class tied to Guzzle, making it harder for people building a client based on another tool (it may be a deliberate decision though)