[CB-861] Header support for FileTransfer download

Added support for an optional options object as the final arg. Currently only handles the options.headers object (as per the issue).

`FileTransfer.download(source, target, successCallback, errorCallback, trustAllHosts, options)`

This is needed for using FileTransfer.download with Basic Authentication, etc. Sadly since Android 2.x doesn't support XHR2, this is needed in FileTransfer.

I have only added support to Android and iOS (see other PR's). It might be that the JS args need to be in a different order or handled differently so that it doesn't impact the other platforms. I am hoping I can get advice on that from someone.

If there is anything else I need to do, just let me know.

//cc @agrieve - for suggesting I do this myself