Update Adafruit_MCP23017.cpp

Allow the source to be used on non-arduino boards, such as the ChipKIT MAX32
- refactored code, common patterns are now private methods
- added support for interrupts
- added support for interrupt configuration
added methods to get information about the interrupt pin and pin value
added interrupt example
Rename .pde to .ino
Add note about reset pin to examples.
Merge pull request #2 from hydra/chipkitmax32

Allow the source to be used on non-arduino boards, such as the ChipKIT MAX32
Merge pull request #6 from husio-org/master

refactoring and interrupt support
Update interrupt example to not use LowPower library by default.
Resolve #3 by adding readGPIO function to read single bank of IO.
Merge pull request #1 from adafruit/master

update