Add some extensions

[phuongnd08]

I'm not sure where did I copy these extensions from, perhaps redis-ruby. These extensions let nest use hash directly. I find them very useful. Do you think we can add them to the gem?

Nest.class_eval do
  def mapped_hmget(*fields)
    Hash[**hmget(*fields)]
  end

  def mapped_hgetall
    Hash[*hgetall]
  end

  def mapped_hmset(values)
    hmset(*values.to_a.flatten)
  end
end