Fix username in api

[mrbriandavis]

These should resolve issues connecting, authorizing and using the hue lights api

Here is where the document explains the change: http://www.developers.meethue.com/documentation/configuration-api

Important bits:

**Important! The username will soon be deprecated in the bridge. It is strongly recommended not to use this and use the randomly generated bridge username.** 

[xavier]

Thanks for the PR! 👍

I hadn't closely followed the latest updates in the Hue API. I just had a quick look at the new authentication mechanism and it seems that to implement the new API properly, we should allow the devicetype to be set and update the documentation accordingly in the README and ExDoc comments. If you have some spare cycles to do this, that would be awesome. Otherwise I'll already merge this patch and work on that part later this week before publishing a new version of the package.

[mrbriandavis]

@xavier - added another commit that lets devicetype to be set. if this looks ok - i will update docs in the next commit.

I also have unit tests in my branch I would like to add as well.

[xavier]

@mrbriandavis Thanks a lot fo this. Feel free to add your tests and documentation and I'll merge it.

[xavier]

👍 👍 👍 Thanks a lot for this!

The Huex.info/1 test doesn't pass because the response is a map, not a struct. It's alright I'll fix it on master.