Add support for multipart uploads

[leontastic]

Resolves #33

Multipart uploads is a major feature missing from this mock S3 server. Without it, it's impossible to test uploading files larger than 5MB using aws-sdk:

const s3 = new AWS.S3()
// ...
s3.upload(params, (err, data) => {})

[n1ru4l]

Awesome exactly what I need 😅
Build is failing due to ancient nodejs version.

[n1ru4l]

I created #79 to update the nodejs runtime.