App sucks battery

[stuartpb]

StepDub's constant accelerometer polling and background running cause it to be kind of a troublemaker with a phone's battery life.

Plan of action:

• See if there's a profiler I can run the app through.
• Add a "sleep mode" with reduced polling when the phone seems to be standing still (for when it's left on on a desk or something).
• Maybe also activate this sleep mode liberally (or just make noise as a reminder that the app is running) when using/backgrounding the ninja shoes.

[stuartpb]

Sleep mode implemented and working as of 6520aa0. I'm not sure if this necessarily fixed the battery life issue (or if it's even the app's code's fault and not, say, a byproduct of building with debug on), but I left it running for a while and then played one stage of Plants Vs. Zombies, and Plants Vs. Zombies had the higher battery use, so it's probably OK to at least some degree.