Switch to Wiring Pi GPIO utility

* switched to GPIO utility (via tjanson/pi-gpioutil) (see #25)
* automatic export/direction change on read/write (see `exportMode` param)
* refactored pinMap, parameter parser, revision detection into separate modules
* readme updated; some examples; more tests
* (did *not* switch to native bindings (#26), but stayed with fs calls for read/write/{g,s}etDirection)

Let me know if any changes are required. Also: This passes all tests and works for me, but should still be tested manually.