Create a SummaryBuilder and add Formats

[reaandrew]

Currently we have JSON and YAML concerns inside of core:

  //SummaryBuilder ...
  type SummaryBuilder interface {
      Write(summary ExecutionSummary)
  }

  //NewSummaryBuilder ...
  func NewSummaryBuilder(format string) SummaryBuilder {
      switch format {
      case "json":
          return &JSONSummaryBuilder{
              writer: os.Stdout,
          }
      case "yaml":
          return &YAMLSummaryBuilder{
              writer: os.Stdout,
          }
      default:
          return NewConsoleSummaryBuilder(os.Stdout)
      }
  }

Instead of this and to keep core clear of any details of format it would be better to have a method called AddFormat(name string, builder SummaryBuilder) . The correct builder would then be looked up from a map at time of execution. The JSONSummaryBuilder would live in a JSON package inside of the serialisation package and the YAMLSummaryBuilder inside the yaml package.

I missed this on the review of #103