MCP23017 read

On a hack day over the weekend I added some read functionality to your MCP230xx code - it worked great. This is that code re-written to be more in keeping with your formatting, but it is untested. To see the working code you can look at the master branch of my fork.

When I get a chance (probably about 2 weeks from now) I'll be testing this version; so I'll post here again when that's done to let you know it works. I'm just submitting this now to save you having to duplicate my effort in the mean time, in case you need the same functionality :)

Cheers!