Fixed a bug where URIs were properly canonicalized in Sig V4 signatures.

Per http://docs.aws.amazon.com/general/latest/gr/sigv4-create-canonical-request.html, item 2, paths are supposed to be normalized & URL encoded, which they were not being before. ``boto`` doesn't currently use this much of anywhere, but this should make the implementation a little more correct.

All unit tests pass & the integration tests for glacier/cloudformation/sqs pass.