Automatically figure out when we should be dropping a fix in bearing node

[irapha]

Currently, we are just using a distance threshhold. Ideally, we'd want to automatically drop a "new_fix" iff we believe it's not improving our current calculation for bearing.

https://github.com/gtagency/buzzmobile/pull/77/files#diff-3d31e05db1c95ca0eece5af0f4f7064eR60

There are many ways this can be done, including setting a decay rate for previous fixes. Or we could do something fancier like a particle filtering algo.

[irapha]

@kpberry this issue prob relevant to the discussion we were having today