v0.8.0

This is a minor release that extends two features of the API for use with the newly developed web reader application. Both pertain to the display of PDF files in the web reader:

1. A proxy endpoint has been added for use when proxying PDF resources is necessary for display. Access to this endpoint is controlled via a CORS header which can be configured via an environment variable
2. The default Webpub Manifest implementation was extended with a conformsTo field in the metadata block to allow the webreader to identify the type of resources contained within a manifest before loading the necessary renderer/reader.