Bit bang mode

Hey Chris, I've made additions to support bit-bang mode on any 2 arbitrary digital output pins.

It's currently controlled entirely through #defines and #ifdefs, which minimizes compiled code size but is kind of inelegant as a library since the end-user would need to actually modify the library to configure the system.

Alternatively, I could change begin() to take a bunch of arguments or overload it with an version that takes arguments. Although then there is a question of what the no-arg begin() should default to, since right now you could take an existing example and run it in bit-bang mode with no modifications to the code.

If you have a preference, let me know and I can alter the interface to match.