Move to using (some) bcm2835 functions

I kept getting `@ fptr_finalize - /sys/class/gpio/export (Errno::EBUSY)` after I would kill the ruby script I was writing, so I looked into things a bit and found that we could use the driver directly for setting these functions instead of using the file system.

```
Finished in 0.01079 seconds
46 examples, 0 failures, 4 pending
```
Ok, sure. No problem.

Doesn't really prove that it works, but is a verification that the code is using the API I expect it to be using.