Add city database & functions

I added the city database and associated functions. I may do additional work to add the ASN database as well for the ISP/Network information at a later date.

For now, I kept all of the code in the single GeoIP.jl file, but may factor each database into its own file if it makes sense later. I also didn't change the Country database, although it might be out-of-date. The file currently in this package is larger than the newer file, and I didn't know what to make of that, so I did nothing.

Any suggestions about coding improvements, please let me know.