Refactored configuration to allow for key to be passed in directly

I use `cloudfront-signer` on Heroku and like to follow their convention of keeping sensitive data out of the git repo using ENV variables.

In this case I would prefer to keep the key encoded as a string stored as an ENV variable on  Heroku, and so would prefer to supply the key directly, along with the key_pair_id as seen in the following:

``` ruby
AWS::CF::Signer.configure do |config|
  config.key = ENV.fetch('PRIVATE_KEY')
  config.key_pair_id  = "XXYYZZ"
end
```