Add support for multipart upload (#160)

[richardkiene]

• Fixes Add support for multipart upload #160, but omits listing multipart uploads

[richardkiene]

@dekobon I created #162 , #163 , #164 to address your points.

@twhiteman I think I've addressed all of your concerns in my latest push.

The failing travis-ci check is unrelated to this commit (it's an images issue) and I've run the test suite on my machine without issue. Furthermore I've verified that the MPU example code works when run against this PR.

Let me know if you have any more changes or if I can merge this in.

[twhiteman]

@richardkiene yep, from the go-lang point of view I think your code is good to go 👍