Start working on issue #13

Initialize integration with Retrofit. Added the compile dependency into build.gradle.
Created "Offer" POJO - A simple POJO representing an Offer.
Created "OffersAPI" - An API to retrieve JSON data and store it as a List of "Offer".
Finished issue #13

Created "Store" POJO - A simple POJO representing an Store.
Created "StoreAPI", method getStores - An API to retrieve JSON data and store it as a List of "Store".
Created "StoreAPI", method getStore - An API to retrieve JSON data and store it as a "Store".
Refactor rename: "OffersAPI" to "OfferAPI".
Created "OfferAPI's", method getOffer - An API to retrieve JSON data and store it as a "Offer".