rasterio should be optional

[ceball]

Currently, rasterio is required at least to run the tests:

$ py.test datashader --verbose
================================ test session starts ================================
platform darwin -- Python 3.6.0, pytest-3.0.5, py-1.4.32, pluggy-0.4.0 -- /Users/cball/anaconda3/bin/python
cachedir: .cache
rootdir: /Users/cball/code/datashader, inifile: 
collected 68 items / 1 errors 

====================================== ERRORS =======================================
_________________ ERROR collecting datashader/tests/test_raster.py __________________
ImportError while importing test module '/Users/cball/code/datashader/datashader/tests/test_raster.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
datashader/tests/test_raster.py:12: in <module>
   with xr.open_rasterio(TEST_RASTER_PATH) as src:
../../anaconda3/lib/python3.6/site-packages/xarray/backends/rasterio_.py:117: in open_rasterio
   import rasterio
E   ModuleNotFoundError: No module named 'rasterio'
!!!!!!!!!!!!!!!!!!!!!! Interrupted: 1 errors during collection !!!!!!!!!!!!!!!!!!!!!!
============================== 1 error in 2.96 seconds ==============================

I'll make rasterio truly optional, but I don't seem to be able to assign issues to myself.

Note: I couldn't install rasterio on OS X using conda - I had to use pip (ContinuumIO/anaconda-issues#2428).