Add web annotations protocol API client

[alastair]

related: #1

There is a web annotations protocol: https://www.w3.org/TR/annotation-protocol/
This is an HTTP interface that understands the structure of json-ld formatted annotation objects. There are a number of tools that use this protocol to save annotations, and also some servers that understand the protocol.
In order to make this library more useful for more people, it'd be nice to add the annotation protocol as another optional client.