0.8.1

• Added rf_local_no_data, rf_local_data and rf_interpret_cell_type_as raster functions.
• Added: rf_rgb_composite and rf_render_png.
• Added toMarkdown and toHTML extension methods for DataFrame, and registered them with the IPython formatter system when rf_ipython is imported.
• New documentation theme (thanks @jonas!).
• Fixed: Removed false return type guarantee in cases where an Expression accepts either Tile or ProjectedRasterTile (#295)