Set no ETag on resources when application provides no ETagAdapter

This introduces backwards compatibility with zope.browserresource before
3.11.0 and allows applications not to use ETags and rely on timestamps
instead without having to work around this by adding a NoETagAdapter as
the test does.
Add Changelog entry for GitHub #1
Merge pull request #1 from rbu/allow-start-without-etags

Set no ETag on resources when application provides no ETagAdapter