StoragePassword/StoragePasswordCollection: Improved tests; found and fixed an API hole; improved documentation.

We now have

	IStoragePassword.GetAsync(stringUsername, string realm = null)
	IStoragePassword.GetOrNullAsync(string username, string realm = null)

and an updated signature for

	IStoragePassword.CreateAsync(string password, string username, string realm = null)

It used to be that username came first in the argument list. This violated the signature in IStoragePassword.

Signed-off-by: David Noble <dnoble@splunk.com>