Add some tests that fail to spot bugs.
Even a host that equals a public suffix may not issue domain wide cookies.

This is described in RFC 6265 5.3.
Edit rdoc.
Officially add AbstractStore#delete() as an API method.
Include MonitorMixin in store classes and make them thread-safe.
Fix handling of the :for_domain option in HTTP::Cookie.new().
Hold session cookies in an internal HashStore.
Store special cookies properly.

"Special" cookies here mean those of public suffixes and IP addresses.
Fully test MozillaStore as well as HashStore.
Elaborate on the hash storage model.
Bump the version to 1.0.0.pre7.