Added Cache Parser Strategy. fixed #821

Now different than before the we store data as plain text, otherwise
we bundle also the marshal serializer or you can simply provide yours.

Example:

    Padrino.cache = Padrino::Cache::File.new(...)
    Padrino.cache.parser # => Padrino::Cache::Parser:Plain
    Padrino.cache.parser = :marshal

Or you can write your own:

    require 'oj'
    module PadrinoJSON
      def self.encode(code)
        Oj.dump(code)
      end

      def self.decode(code)
        Oj.load(code)
      end
    end

Finally you can load your strategy:

    Padrino.cache.parser = PadrinoJSON

Or if you prefer do something on the fight you can:

    Padrino.cache.parser = Module.new do
      def self.encode
      end
      def self.decode
      end
    end

//cc @padrino/core-members