Add ErrorType

Errors in this release need not be of type NSError. They can be of any type you like (as long as the type is marked as conforming to ErrorType).

This is mostly backward compatible with the NSError-only 0.1.0.

You should expect ErrorType to go away in 0.3.0. We will likely either go back to requiring NSError or move to a fully-parameterized error type (Result<T,E>). This solution has turned out to possibly be the worst of both worlds.

This release also adds a PodSpec. Be very careful building important things on this. That leading 0 in the version is not kidding around. But I do expect things to settle down soon.

This release bumps us to iOS 8.0 so that libraries work correctly.