Adding the old 'signature_version' (HmacSHA1) order to choose between both.

I've implemented it because new signature (SHA256 based) doesn't work (Unauthorized error) with both the latest Eucalyptus (3.3.0) and OpenStack (2012.1.3-dev).
Maybe a bad implementation on the part of the latter two, because I guess it works in Amazon (not trying).
Another interesting thing, the ec2tools commands works fine with the new signature on the two clouds (Eucalyptus and OpenStack)..
When I have time I would look in detail how ec2tools sign requests but for now I use the old signature which works very well (though less secure).