Tussle resolved

I can now upload files larger than a single part, after patching some low-level buffer memory bugs that occur in-browser when passing `slice()` views of Buffers around. Phew!

This brings the supported file size of this project from **1MB** to **1GB**!

Next steps:

* Properly patch [`s3-upload-stream`](https://github.com/nathanpeck/s3-upload-stream) for the browser.
* Test out AWS SDK over npm, rather than freezing it into the repo. If that doesn't work, nudge them to support browserify.
* Figure out how to limit the file reading to keep roughly in sync with the S3 upload stream, so that a constrained window of memory is in use at all times. This will be necessary to raise the file size limit.