Added support for YAML serialization/de-serialization of Spira::Base inst

[gkellogg]

Added support for YAML serialization/de-serialization of Spira::Base instances using Psych.

I basically fixed the serialization I was doing in sparql-tests, and made it more generic. The only other change I found useful was to implement #encode_with and #init_with for RDF::URI, as otherwise Addressable is serialized. This is what I did there. Maybe it should be in Spira too.

# Save short version of URI, without all the Addressable stuff.
class RDF::URI
  def encode_with(coder)
    coder["uri"] = self.to_s
  end

  def init_with(coder)
    @uri = Addressable::URI.parse(coder["uri"])
  end
end