Use static_url template filter for footer image.
Use static_url template filter for footer image.

Fixes #634

kdeloach/nyc-trees,azavea/nyc-trees,maurizi/nyc-trees,maurizi/nyc-trees,RickMohr/nyc-trees,RickMohr/nyc-trees,azavea/nyc-trees,azavea/nyc-trees,kdeloach/nyc-trees,RickMohr/nyc-trees,kdeloach/nyc-trees,kdeloach/nyc-trees,RickMohr/nyc-trees,azavea/nyc-trees,kdeloach/nyc-trees,maurizi/nyc-trees,azavea/nyc-trees,maurizi/nyc-trees