Implement caching logic for directions

[jkwening]

Figure out caching protocol for directions - I'm thinking prefetch and cache tiles at varying zoom levels traversing directions waypoint; then you can use user location to center directions map and thus pulling nearby tiles. We will need to figure out how to best handle off-course during offline mode.

[motosharpley]

@jkwening one thing I noticed looking at this tonight too is that when the marker is moved while offline the directions pane is lost so we may need to figure out a way to either preserve that or prevent the update/action when offline