Implement Upload

[mwinche]

@bhovhannes I love what you've done so far. I find that I need to make use of the upload feature to attach some documents to some issues for my script.

I'm assuming that you're okay with it as you have a stub out there. How would you recommend I proceed? Or should I just start working on a PR?

[bhovhannes]

@mwinche, document upload feature was also mentioned multiple times in stackoverflow and certainly is a feature the customers will love. I was also thinking about implementing it, just didn't managed to find a time to do that. If you'll have time to work on implementation, I'd like to accept your PR :)

I've some thoughts regarding how it should be implemented:

1. A new file named upload.js should be added to src/plugins/ folder. It will contain method for uploading file and getting its handle.
2. Document new method with JsDoc. JsDoc comment will be used to generate API docs afterwards.
3. The file upload.js should be excluded from browser bundle, because upload can't work in browser-based environments. In order to exclude it, tweak build task in gulpfile.js.
4. Make sure to add an example in examples/node/ folder illustrating usage of new method.
5. I was thinking about using form-data package, but if you have a better choice, feel free to use it.

[mwinche]

Thanks @bhovhannes! I'll start working on it now!