Add NewIOTransport function

[joshcarp]

Adds new NewIOTransport function that returns a Transport.
I needed this because I'm reading/writing to a custom file descriptor.

[jba]

We'd probably also want to export the IOTransport type and return that directly, in case we want to add exported fields or methods to it.

This sounds reasonable, but we're going to discuss it internally a bit, as we would with most new API. That may take a couple of weeks due to vacations.

[joshcarp]

Yeah i noticed this repo isn't returning interfaces, so as a convention it's probably a good idea to make IOTransport public too

[findleyr]

See also #376. I think this is reasonable, but we need a proposal.