Implement sendMultipartPostData method

Hi, i'm currently refactoring a legacy software that is using v1.x of this client and i can't migrate to v2 (available via composer) because method sendMultipartPostData has been removed.

Instead of creating a new fork, if this repo is still active, I could write and make a PR for a new sendMultipartPostData method using CurlFile for file upload, requiring PHP 5.5+.

