Multiple Compressor support

Ok, here is an implementation in order to support multiple compressor.

To add a new one, juste create another compressor with this skeleton:

```
require 'passbook/compressors/base'

module Passbook
  module Compressor

    class COMPRESSOR_NAME < Base

      ###
      # Human-readable name of the compressors
      #
      def name
        "COMPRESSOR_NAME"
      end

      ###
      # Output zip string
      #
      def outputZip pass, files, manifest, signature
        # Your compression process
      end

    end

  end
end
```

Do not forget to run tests and add the compressor name to `all_compressors` in `lib/passbook/compressor.rb`