Use ReadableByteStream/fetch for supporting browsers (Chrome). Close #7.

[ariutta]

All tests pass.

Only things I noticed: in Chrome, the "request ended" message sometimes shows twice, with the first time coming before the final chunk. In FF, there seems to be a delay between the second and the final chunks.

There are also several TODOs in the code, and I made what I thought were some corrections, but it would be best if you could check my changes to be sure.

[ariutta]

Would be good to verify that requests.destroy() does what's expected with this change.

[coveralls]

Coverage remained the same at 42.857% when pulling 49d2a24 on ariutta:master into 87d2fa1 on unshiftio:master.

[coveralls]

Coverage remained the same at 42.857% when pulling f82feae on ariutta:master into 87d2fa1 on unshiftio:master.