Add nullability annotations on to PropertyProvider interface to highlight incorrect usage.
Fix bug in AbstractManifestServlet which results in a NullPointerException
when a provider does not return a value. An example where this occurs is
when a client-side property provider can also be derived on on the
server-side when a cookie is set.

Reported by Paul French.

Fixes #8
Remove pending flag
Update version in the readme
Prepare for next version