[AWS|Storage] allow signature version to be set to 2 (WIP)

I've had a stab at this, just for main requests (doesn't support presigned urls yet). There's some fiddly stuff going on - signature v4 requires that you do canonicalisation of path/host etc before you sign, signature v2 behaves differently, signature v4 requires that you regenerate the signature when following a redirect but signature v2 doesn't, so there are quite a few possible combinations to test.

I think the other amazon services would be a lot easier to add support for multiple signature versions to.