Optionally provide margin of error for acs data

Function get_ca_tract_acs_data now takes a parameter include_moe which, if true, will return a column with 90% margins of error.

refrence maps

setting reference_map=TRUE when calling ca_tract_choropleth will result in the choropleth being rendered over a reference map from google maps.

Initial release

v1.0.0

changes for cran