Refactor to avoid unneeded numpy imports.

Closes #140.

Fairly easy because Numpy methods accept either strings like
`'uint8'` or types like `np.uint8` as dtypes. So do Rasterio
methods now.
Merge pull request #141 from mapbox/issue140

Refactor to avoid unneeded numpy imports.
Switch to console scripts entry points.

For #137.
Migrate 3 legacy scripts to entry points.