indicating progress

[st-h]

This ticket is similar to the already closed #53, however as ember is moving towards making jQuery optional, we are going to need a feature complete replacement for making XmlHttpRequests. Currently the lack of displaying progress is blocking some people from using ember-fetch as such a replacement. Even though it seems that progress will not be supported when the polyfill is used, I think it is important to add this feature in order to allow projects that target up to date browser to make usage of this addon.

I can put some time into putting together a PR, but as there are other aspects involved like #63 or #45, I think it is best to agree on an approach and details first.

[nlfurniss]

Unfortunately, unless the fetch spec allows for indicating progress, this is something you will have to continue to use XmlHttpRequest for.

[xg-wang]

For more info on spec:

• upload progress? whatwg/fetch#21
• FetchObserver (for a single fetch) whatwg/fetch#607