Feature/import existing data

This PR adds a rake task to do a onetime import of mdcWetap.csv data. You can view the CSV downloaded from the google fusion tables here:
https://s3.amazonaws.com/wetap-development-resources/mdcWeTap2.csv

to run the import execute:
`be rake wetap:import_existing_data INSERT=1`

For repeated runs, you can use the `CACHE=1` env variable.

New columns are added to support the data sources and associated keys. This should be helpful in merging in e.g. new OSM data (if one were to do that).