Fix for submitting multiple recipients to a list simultaneously.

Hi NeoSeekers - thanks for putting in the work to make this library PSR-0 compliant and Composer-loadable. Much appreciated!

Yesterday I spent some time working with SendGrid Support, trying to find out how I could load multiple users into a recipient list simultaneously. This is documented in the Newsletter API Documentation, under the curl examples.

Problem is that SendGrid wants us to submit the same parameter multiple times (data[]). Using numeric indexes in the array does not work, so what I ended up doing is manually building up the HTTP PostData querystring.