Sphero handleCollisionDetected data structure.

Delivers the data payload for sphero collisions as a struct rather than raw bytes. 

A `.(sphero.Collision)` type assertion is necessary to access the fields. A `comma, ok` check is recommended in case the collision is missing the data (which I've seen at times).

![sphero-collisions](https://cloud.githubusercontent.com/assets/4566/3943754/4c782eda-25c7-11e4-8f67-85b0abaf9564.png)

this checksum code should be moved down to apply to all responses sent to us.