Http multipart + remoting multipart + neko/php web multipart params support

-haxe.Http now supports multipart encoding. addFileTransfer function added, files support on all plateforms
-neko.Web and php.Web getMultipartParams function added that is a StringMap of Strings, { filename : String, bytes : haxe.io.Bytes } or Array<{ filename : String, bytes : haxe.io.Bytes }> (if multiple file upload with same name)
-Multipart remoting supported like that : if a param is { param, filename, bytes, ?mimeType } then all the transfert is delegated to multipart/form-data (simple String params too). The requests are handled checking also against multipartParams and not only url-encoded params ( __x ). If files are found, they are transmitted as an array of files params, as function arguments