use setuptools

Handles dependency installation, which now must be listed in setup.py

Adds setuptools bootstrap module
clean-up .travis.yml
deeper integration of CRS into Grid types

Grid.get_indices now assumes geographical coordinates and uses Grid.crs to
compute the proper coordinates

Grids without a known CRS are associated with an IdentityCRS in which coordinate
map to themselves under projection. This might not be the best way to do it, so
consider this change unstable for now.
Merge branch 'grid-crs' of github.com:njwilson23/karta into grid-crs