remove custom event
add progress-stream
fixes upload cropping
make synchronous re #472 and return a progress-stream
use request for multipart upload
set custom content-length header
bring back callbacks, remove events, end -> finished
update test for progress-stream
Hand-merge of pass-errors branch.
Stray task references.
Merge pull request #4 from mapbox/source-upload-pass-errors

Merge of pass-errors branch
form-data fixes unpipe
fix stream scope
update tests for new multipart form structure
return error to end request when canceled
update request, clarify stream length
fix error handling
use getCustomHeaders() instead of getHeaders() so content-length is not set manually
update readme
update request