Pyplant

- It is now possible to extend PyPlant with custom ingredient types.
- Subclass `IngredientTypeSpec`, implement abstract methods and register with `plant.warehouse.register_ingredient_specs`.
- HDF-arrays, Keras models and SciPy sparse matrices are now ingredient specs, available in `pyplant.specs`

- No longer have to provide an ingredient type, when sending it.
- Pickled NumPy arrays are now allowed.
- More robust against data corruption.
- Improved unit test coverage.

Published on PyPI.
Wrote a proper introduction guide.
Cleaned up some names.