Use cipher-aes' API instead of using it via crypto-api.

This commit should increase clientsession's performance, but I
have not benchmarked it.

The only API breakage is changing IV from a type to a newtype.
@snoyberg, do you think we should bump the major version?
Merge pull request #18 from meteficha/use-cipher-aes-api

Use cipher-aes' API instead of using it via crypto-api.