Decorators, multiple headers, and better error messages

Store response headers as a list since a HTTP response can have the same header twice (happens with set-cookie sometimes). This has the added benefit of preserving the order of headers. Thanks @smallcode for the bug report leading to this change. I have made an effort to ensure backwards compatibility with the old cassettes' header storage mechanism, but if you want to upgrade to the new header storage, you should delete your cassettes and re-record them. Also this release adds better error messages (thanks @msabramo) and adds support for using VCR as a decorator (thanks @smallcode for the motivation)