Pca95x4 should implement GpioDriver

[shaggygi]

Pca95x4 is an IO expander but it doesn't implement GpioDriver. We should provide this for consistent access

[JeremyKuhne]

Note that I've got a new library with Pcx857x support in it that I hope to get a PR open for today.

[shaggygi]

Another side note... we might want to review #135 at some point since expanders support other pin modes in addition to what the current core API includes. For example, a couple of the MCPs have open-drain outputs. This might be a good exercise to do sooner than later as I noticed there are some places in the core GPIO that would have to be updated, as well.

[JeremyKuhne]

#229 is the initial Pcx857x PR.

[Ellerbach]

Rather than IGpioController which won't really happen, this should then implement the board abstraction and propoer a real GpioController.