Added postal code column

This PR is in regards to the issue on the previous issue I opened on the main project https://github.com/openaddresses/openaddresses/issues/378.

I went ahead and implemented support for zip codes as a start. This is a bigger change since it would require basically all sources to be updated (which I'm looking into doing). So please feel free to tell me if it's not appropriate to merge in right now. No feelings will be hurt :)

## Implementation details
I decided to call the field "Postal Code" rather than ZIP Code since it's more international and due to the fact that "ZIP Code" eligibly *used* to be [trademarked by USPS](http://tsdr.uspto.gov/#caseNumber=73001312&caseType=SERIAL_NO&searchType=statusSearch).

Due to the existing implementation of CSV columns, I decided to implement it so the rows in the *POSTALCODE* column simply will be blank if the data source doesn't have *postal_code* defined in the *conform* section.

## Example source with postal codes
```json
{
    "coverage": {
        "country": "us",
        "state": "va",
        "city": "Arlington"
    },
    "data": "http://gis.arlingtonva.us/arlgis/rest/services/public/Addresses/MapServer/0",
    "type": "ESRI",
    "fingerprint": "33b698696913a6b60484f1b7940ed9ed",
    "version": "20140814",
    "cache": "http://s3.amazonaws.com/openaddresses/20140814/us-va-arlington.json",
    "conform": {
        "lon": "x",
        "lat": "y",
        "number": "stnum",
        "street": "stname",
        "postal_code": "zip5", <-- This is the new column
        "type": "geojson"
    },
    "processed": "http://s3.amazonaws.com/openaddresses/us-va-arlington.csv"
}
```