Add initial support for Kobo device Sync endpoint.

- Supports /v1/library/sync call to get list of books
- Supports /v1/library/metadata call to get metadata for a given book
  + Assumes books are stored on Backblaze for metadata call
- Changes to helper.py so that we can return no cover instead of a blank
image.

cps.py

@@ -41,6 +41,8 @@ from cps.shelf import shelf
 from cps.admin import admi
 from cps.gdrive import gdrive
 from cps.editbooks import editbook
+from cps.kobo import kobo
+
 try:
     from cps.oauth_bb import oauth
     oauth_available = True
@@ -58,6 +60,7 @@ def main():
     app.register_blueprint(admi)
     app.register_blueprint(gdrive)
     app.register_blueprint(editbook)
+    app.register_blueprint(kobo)
     if oauth_available:
         app.register_blueprint(oauth)
     success = web_server.start()