Manage request as an upload

[Westbrook]

This is also in reference to #235.

This is a first pass at this PR as I have a number of questions about what form a more complete implementation should be.

Here I've added an isUpload property to both the iron-ajax and iron-upload so that when it is set to true the listener in iron-request that updates the progress property is attached to xhr.upload instead of just xhr. This allows the implementer to track file uploads into the UI, etc.

First, I'm wondering if when uploading all events tracked in send() should be pointed to xhr.upload. Second, should this actually be an even greater change where the progress object be duplicated into uploadProgress and downloadProgress so that both objects and listeners can be tracked at all times?

Once that is figured out, I'd look for insight on good ways to add tests for this. There doesn't seem to be any test in iron-request for progress, and this seems like it might be a good place to do so?

[Westbrook]

Any update on this?

[Westbrook]

Ping?

[btelles]

@Westbrook FWIW, I implemented this on my own and for some reason the progress property is not propagating up to the iron-ajax element. very weird.

[Westbrook]

@btelles Implemented from my fork? Did you instal via bower.json? I had a pita of a time getting bower to recognize that as the version to use vs the ones pointed to in other dependencies. If you are working from my code I'd be glad to hear your experience and see if we can't find the right changes to support a wide set of users for this. I actually just released my feature based on these changes this morning, but I could have easily been working around something on accident in my own code.