Rough initial commit to prove that we can get an ISR and service it

Should handle the ISR for rising or falling and stuff the level (for
now always 0) into a buffer.
Added pin toggle for debugging and example sketch
Added interfaces to poll and to copy buffer when data is available

Still needs testing on hardware.
Fill frame buffer with actual state
Added use of new interfaces to example sketch
Moved example sketch to single depth
Added some code to detect when we overflow our frame buffer or write over a frame without reading it
Refined implementation using Timer1 Input Capture.  Refactor to allow for streaming or buffered decode

Next steps must include:
 * Better documentation
 * More testing
 * Example decode

Could include:
 * Pull buffered decoder out and make it an example instead.
 * Optional noise cancel
 * Adjustable prescaler?
 * Example stream decoder.
removed .ds_store file. Need to add to gitignore!
Merge branch 'develop' of https://github.com/racerx/BTHI_IR_Decoder into develop
Added .DS_Store to the git ignore file