Add `hash_key` option

1. Add gemspec
2. Add tests
3. Add more information on README.
4. Finally, add `hash_key` option. 

Basically, what I wanted to do is to forward messages whose tags are like

    foo.bar.host1
    foo.bar.host2

to the same node. To do it, I had to modify the hash key to `foo.bar` by eliminating the host information. With the new `hash_key` option, we now can achieve the goal by specifying as follows:

    hash_key ${tags[0..-2]}

Default is the same with `${tag}` (original behavior). 

PS. Could you release gem? Or, could you add me to collaborators?