Added an odometry tracker, and a /stopAfter URL that stops the vehicle once it
has traveled a specified distance.

For example, this sequence instructs the vehicle to move forward 1 meter:

curl 'http://127.0.0.1:8080/front/0.1'
curl 'http://127.0.0.1:8080/stopAfter/1000'
Merge branch 'master' of github.com:wiseman/ar-drone-rest
Added /stopAfter example to the README.