Refactor datasets to use slippy map dataset abstractions

[daniel-j-h]

At the moment we still have our old dataset classes not yet working with the slippy map directory datsets:

https://github.com/mapbox/robosat/blob/52299b855b77225b795b015944e31cbddb9ce44b/robosat/datasets.py

Tasks:

• remove the ImageDirectory dataset as it is unused
• implement the BufferedSlippyMapDirectory on top of the SlippyMapDirectoryDataset