Allow user to attach the document via URI

[peerchemist]

As described in the perpera lib documentation:

It is possible to attach URIs to a document:

doc.addUri('https://example.com/foo.txt', spender).then(() => {
  // the transactions have been sent to the network
});

If the given URI is too long to fit into a transaction, an error is thrown.

[peerchemist]

This is a first step toward:

#8

and

#7