Add path by mph

This now allows you to generate random GPS paths for an object moving
at a certain speed with updates at a certain frequency. This results in
more realistic data.
Merge branch 'master' of github.com:wiseleyb/spot_tracker

* 'master' of github.com:wiseleyb/spot_tracker:
  WIP Path by mph

Conflicts:
	app/models/data_generator.rb