Add conversion method ISO3166::Country()

Implement ruby's [conversion idiom](http://www.ruby-doc.org/core-2.1.3/Kernel.html) for ISO3166::Country class, so:

```ruby 
country = ISO3166::Country.search('US')

ISO3166::Country(country) #=> #<Country:0x007f9c61e0ea20 @data={....}>

ISO3166::Country('us') #=> #<Country:0x007f9c61e0ea20 @data={....}>

ISO3166::Country(42) #=> raises TypeError, "can't convert Fixnum into ISO3166::Country"
```