The #key? method should be supported

In fact, the `Hash#has_key?` has been (informaly) deprecated by `Hash#key?`
cf. http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-core/43765
so we should have a `Store#key?` method, and an `Store#has_key?` alias.
Merge pull request #58 from jlecour/key

The #key? method should be supported