Schematron releases should use semantic versioning

[tgraham-antenna]

@phax commented in #55 that Schematron should use semantic versioning for releases.

I agree that semantic versioning should be used.

The current single release -- https://github.com/Schematron/schematron/releases/tag/2017-02-09 -- exists because we needed a baseline against which later changes can be compared. It is tagged with a date because @rjelliffe is the only one with the meritocracy level to say what the Schematron version number should be.

[phax]

Great - sounds good :) What does "meritocracy level" mean?

[rjelliffe]

Yes. The issue goes beyond versioning: should we have separate distros and code bases for xslt1, 2 and 3 engines? That would allow the XSLT 2 & 3 versions to be single scripts not a pipeline.