Return query size

This is an utility method. It isn't hard to realize that
`query.key.llen` will return the current size of the queue, but this
makes it even easier.
Merge pull request #4 from inkel/queue-size

Return query size
Fix pop/each usage docs in README

Signed-off-by: Michel Martens <michel@soveran.com>
Update CHANGELOG.
Merge pull request #5 from frodsan/patch-1

Update CHANGELOG.