Move this._layer initialization to _initialDraw

This fixes a bug where, when the location filter was added,
removed, and re-added to a map, it would not display. This
happened because the re-add would reinitialize this._layer
but not call _initialDraw, which sets up this._layer.
Merge pull request #14 from jfirebaugh/readd

Move this._layer initialization to _initialDraw