Added interrupt support, including GPIO interrupt export to userspace and blocking input read method.  Commented example program and included simple example efficient wait for input method.
Moved interrupt support to mcp23s17 library in order to facilitate sharing with PiFace Control & Display module or other mcp23s17 based systems.  Added method to allow users to disable interrupts.
Merge pull request #2 from scm6079/master

Added interrupt support with enhanced & commented example