make call to mg_url_decode() optional

Certain backends need to have the raw request, e.g., for checking
request signature. Make the call optional.
Added a new boolean "decode_url" config param.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
Merge pull request #32 from yehudasa/master

make call to mg_url_decode() optional by adding a new configuration