WALT firmware for Adafruit ADXL335 board

Love the WALT tool! Experienced some frustration with the Tap Latency functionality though, until I discovered that the WALT firmware wasn't actually getting any data from the Adafruit ADXL335 accelerometer board. The firmware only targets the pins for the on-board ADXL335 accelerometer; the Adafruit ADXL335 accelerometer board maps to different pins on the Teensy arduino processor.

I recommend one of two solutions. Either:
- connect the Adafruit ADXL335 accelerometer pins to the same pins as the on-board ADXL335; this has the drawback of making those GPIO header pins unusable for anything else though.
- change the firmware so that the user can select one of the two options with only minimal effort. Attached is an update to the walt.ino file that uses a #define to select one of the two modes. This still requires the user to make the change and upload the correct firmware though, so perhaps there's a better solution, like storing both binary versions on the phone/tablet and allowing the user to select which one to upload, or something like that...

Thanks,
Jason

[walt.txt](https://github.com/google/walt/files/997315/walt.txt)