Added first example of borders information (from Afghanistan to Antigua)

As i wrote in the issue #16 in this commit I've added my idea to get the neighboring countries of a specified nation..

I've added a list "borders" in countries.json entry. I've made these changes only from Afghanistan to Antigua.

I've used the CCA3 as key in the borders list, and as @mledoze wrote in #16 he will add the borders information in his project (we will see if I may help in this operation).

So I submit this PR as a possible base of work.

Thanks.