Gender.io multiple requests

Hi Kasper,

I was playing around with Gender.io today and came across your gem. Thanks for writing this -- it makes it much easier to play around with.

I wanted to be able to send multiple requests to Gender.io.  I've updated the gem to allow an array of names and hope that you will find it suitable to add to the gem!

I also cleaned up a few issues I encountered when running the specs:

In Ruby 2.0 there is an issue loading JSON::Parser. Fixed this issue

Specs weren't passing for me initially because the output was changed to accommodate ActiveSupport caching. Changed the specs so that they are passing now

I've put each change in a separate commit -- feel free to cherry pick as you will or I can cherry pick and make a new pull request.

More details are available in the git log.