Lookback

[dnomadb]

First implementation of lookback - specifying a number of zoom levels to "look back" to if a loaded tile is not found.

cc @yhahn

[coveralls]

Coverage increased (+0.06%) to 96.705% when pulling fcfefd5 on lookback into f7e4ad6 on master.

[coveralls]

Coverage increased (+0.1%) to 96.591% when pulling 290385d on lookback into e282cee on master.

[coveralls]

Coverage increased (+0.1%) to 96.591% when pulling e7eca4c on lookback into e282cee on master.