Makes two Country instances compare as equal if they have the same data

[fj]

Currently, Country instances cannot be compared to each other because performing two different searches yields two different instances. That is, you get the following very surprising result:

Ψ irb
ruby-1.9.2-p290 :001 > require 'countries'
 => true 
ruby-1.9.2-p290 :002 > Country['US'] == Country['US']
 => false 

This pull request fixes that problem. A spec is included.

As a future idea, you may also want to consider making Country instances immutable and caching the instances, since Country[x] should be an idempotent operation.

[saturnflyer]

+1 and I agree with making the [] call cached and idempotent