Document metadata

This pull request adds support to the annotator-store to persist document metadata with annotations, and also as a separate ES index `/annotator/document`. The new index is used when looking up annotations, so that we also look for annotations of representations at equivalent URIs. This is useful when used with the Document plugin that was recently [merged](https://github.com/okfn/annotator/pull/208), which will extract metadata and send it along with an annotation. annotator-store will continue to work as before if the Document plugin is not enabled in the annotator client. This code is related to work we have been doing at Hypothesis on [cross-format-annotations](https://github.com/hypothesis/h/wiki/cross-format-annotations).