Improve `httpcompression`

- correctly detect `SDCH` compression, see:
  http://www.blogs.zeenor.com/wp-content/uploads/2011/01/Shared_Dictionary_Compression_over_HTTP.pdf
- correctly overwrite the user agent (use the -A, not the -H option)
- limit the connection and operation time (useful for preventing curl
  from hanging for hours due to slow networks or links going down)
- show errors
- show redirects