Respect declaration order when printing summary columns

[nschneid]

Currently the summary variables are alphabetized, but the rationale for this is unclear; I would prefer if the order in the output declarations would be used. For instance, in:

summary xresults {
  of learn > NLabels NParams {
    ...
  }
  of eval > Acc P R F {
    ...
  }
}

the order would be: NLabels, NParams, Acc, P, R, F

[ghanneman]

I would like to +1 this bug :-)