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
```
Example Spec
```ruby
describe PiPiper:: Bcm2835 do
  describe '.pin_outpub(pin)' do
    PiPiper::Bcm2835.expects(:gpio_select_function).with(42, 1234)
    PiPiper::Bcm2835.pin_output(42)
  end
end
```