Added ForeignKeys to the Entities

This is used to link the object to its corresponding higher level object
(ex. Province belongs to Country, so the ProvinceEntity has a property
for CountryKey)
Added Find Methods

Implemented Finding an object by it's parent or child. (ex. You can what
continent a Province is in by using the method
ContinentEntity.FindByProvinceKey(string provinceKey)
Added Continent Object and Interfaces