checking in some utilities which fetch census data from the cloud

also a notebook which demos how this works
pep8
tested for whole county of data - works fine
adding unit tests for census helpers (100% coverage)
pep8
add census to the list of dependencies
I think I needed census>=.6
make numexpr a dependency
census .5 is the latest?
I think I figured out how to use census .5
pep8
Merge pull request #5 from synthicity/census-cloud

helpers to get census data into data frames from the census rest api