started ProofML project, uploaded xml-schema

xml-schema is valid with W3C!
separated logical/material proof schemas; added sample instances

todo: ProofType still
major changes to xml-based proofs

- changed comments to summaries
- allowed math in summaries
- removed summaries (except parent element's summary) from Logical
theorems
- renamed ProofML folder to xProof
-added example XML instances

todo:
- decide where  model and isAxiom attributes go
- ProofTypes!