Check if the scheme is http(s) and the host is non-nil in URI.
A Store class should define empty?.
Make each(uri) not fail if a non-HTTP URL is given.

Fix documents with #each.  The uri argument was not made a keyword
argument.

Guarantee that the +uri+ given to AbstractStore#each() is a URI
object.

Make HashStore#each return self as required.
Bump VERSION to 0.1.1.