[FIX] Convert  XTandem search results into .pepXML by IDFileConverter

Currently, converted pepXML in OpenMS only stores PeptideProphet results but not PeptideID information.
Fixes #809
I should add a TODO here. I was sure TPP needed at least one more type of scores, excluding expect score. I found OMSSA only storing e-value as peptide hit score and throwing away all the other scores. Now if TPP doesn't support OMSSA, can we just remove all the OMSSA part?