= Multimap

A Ruby multimap implementation that also includes multiset and nested multimap implementations.

== Example

  require 'multimap'

  map = Multimap.new
  map["a"] = 100
  map["b"] = 200
  map["a"] = 300

  map["a"]                              # -> [100, 300]
  map["b"]                              # -> [200]
  map.keys                              # -> #<Multiset: {"a", "a", "b"}>
