Improve `httpcompression`

**Improvements:**

 - correctly detects `SDCH` compression according to the [specification](http://www.blogs.zeenor.com/wp-content/uploads/2011/01/Shared_Dictionary_Compression_over_HTTP.pdf)
 - correctly overwrites the user agent (use the -A, not the -H option)
 - limits the connection and operation time (this is useful for preventing the function from hanging for hours due to slow networks or links going down)
 - shows errors
 - shows redirects

**Tested on** Mac OS X v.10.8.2 and Ubuntu 12.04 LTS 

---

**Examples:**

<table>
<thead>
    <tr>
        <th> input </th>
        <th> output [old]</th>
        <th> output [new]</th>
   </tr>
</thead>
         <tr>
             <td><pre>adobe.com</pre></td>
             <td><pre>adobe.com is not using any encoding</pre></td>
             <td><pre>adobe.com 
 ↳ http://www.adobe.com/ [gzip]</td>
</pre>
         </tr>
         <tr>
             <td><pre>"http://www.google.com/s?q=alrra&output=search"</pre></td>
             <td><pre>http://www.google.com/s?q=alrra&output=search is encoded using gzip</pre></td>
             <td><pre>http://www.google.com/s?q=alrra&output=search [sdch,gzip]</pre></td>
        </tr>
         <tr>
             <td><pre>firefox.com</pre</td>
             <td><pre>firefox.com is encoded using gzip
Content-Encoding: gzip
Content-Encoding: gzip
Content-Encoding: gzip</pre</td>
             <td><pre>firefox.com 
 ↳ http://www.firefox.com/ 
   ↳ http://www.mozilla.org/firefox/ 
     ↳ http://www.mozilla.org/en-US/firefox/new/ [gzip]</pre</td>
         </tr> 
         <tr>
             <td><pre>google.com/+</pre</td>
             <td><pre>google.com/+ is encoded using gzip</pre</td>
             <td><pre>google.com/+ 
 ↳ http://www.google.com/+ 
   ↳ https://plus.google.com/ [gzip]
     ↳ https://accounts.google.com/ServiceLogin?service=oz&continue=https://plus.google.com/?gpsrc%3Dgplp0&hl=ro [gzip]</pre</td>
        </tr>
        <tr>
             <td><pre>sadsada.dsa</pre></td>
             <td><pre>sadsada.dsa is not using any encoding</pre></td>
             <td><pre>curl: (6) Could not resolve host: sadsada.dsa; nodename nor servname provided, or not known</pre></td>
        </tr>
</table>