Implements the ability to POST uploads

[jonathanberube-zylo]

This PR adds a backwards compatible mechanism to allow for POST uploads.

See a tutorial for how to implement the server, but the gist is replace the existing getSignedUrl call with createPresignedPost. For example:

const data = s3.createPresignedPost({
  Bucket: process.env.BUCKETNAME,
  Fields: {
    key: getRandomFilename(), // totally random
  },
  Conditions: [
    ["content-length-range",   0, 1000000], // content length restrictions: 0-1MB
    ["starts-with", "$Content-Type", "image/"], // content type restriction
    ["eq", "$x-amz-meta-userid", userid], // tag with userid <= the user can see this!
  ]
});

This allows significant more control over what kind of content that clients can upload by enforcing it from your server. For example restricting the file size (as mentioned on #64, #230).