New Env class and tests.
More logging, tests, safer environment
Remove usage (deprecated) of rasterio.drivers()
Also pep8 cleanups throughout the tests and a fix for unchecked
dtypes when opening a dataset in 'w' mode.
Now we can simply import warnings in __init__.py. In the
deprecations tests, we needed to see a single warning only one
time to avoid multiple drivers() warnings.
Add a global env.
Add rasterio.env.setenv()
