Split files into library structure.

[pauldemarco]

As per request by @szakarias in PR #531.

This does not include any changes from #531 regarding upload monitoring.

[collinjackson]

lgtm, we should add missing dartdoc but that can be done in a separate PR.

I'm on the fence as to whether UploadTaskSnapshot should have its own file

[pauldemarco]

@collinjackson I agree, I did that in anticipation of #531

Let me know if you'd like me to combine this back in for now.

Edit: Ah you're referring to the snapshot, I think that would be a good idea. iOS uses a FIRStorageTaskSnapshot for both uploading and downloading, Android uses a UploadTask.TaskSnapshot for uploading and a FileDownloadTask.TaskSnapshot for downloading, but I think it's safe to say we can combine on dart side.

[szakarias]

LGTM, I'm fine with keeping the upload_task.dart file as it is.

[collinjackson]

I've merged this, I think there's no need to release this as a separate version since the changes aren't visible to the developer but please do mention it in the CHANGELOG for #531