A truncated exponential backoff for csma

Implement a truncated exponential backoff for csma, with an easily configurable maximum exponent. I believe the current implementation uses a linear backoff for historical reasons only. Exponential backoffs are known to better adjust to the traffic load. In experiments in the Indriya testbed (http://indriya.comp.nus.edu.sg/) this exponential backoff helped increase the overall reception rate and reduce energy consumption.