Add getMultipleCountries Method

- Added getByCodeList method to CountryService.java. This method provides a way to get a set of JSON object (Countries) by passing a list of alphacode to the getMultipleCountries method of CountryRest.java.

Is it possible to call the rest method in this way: /multiplecountries?countrylist=ar;be;fr;it

The method will return the countries (if existing) associated to the particular code specified in the QueryParam.

- Added a Unit Test file for the specific method: CountryRestMultipleCountriesTest

I hope this should be helpful. i think this method can be a base to make avalaible more complex way of searching using the RESTCountries.