modified Unirest::request; added Unirest::_filterAtFileAttachements

Since PHP 5.5, the "@/path/to/file/attachment.ext;type=mime/type" syntax is deprecated in favor of using the CURLFile class.

This commit adds a check before setting CURLOPT_POSTFIELDS to convert "@/file"-style attachments to CURLFile objects instead (if the class is available).  This is for backwards-compatibility only; it is preferable (and faster) to refactor your application code.