fixed gc for Timespans and StreamStatus

Garbage collection requires weak refs
added registration for subconverters
try new base.getContextByClass
update test results

one error is due to the temporary commit of a function not 100% working…
fixed cached time in seconds
NEW: mei/__init__.py to Start MEI Development
WIP-MEI: add 'mei' to converter.Converter.validHeaderFormats

This is for the formatFromHeader() method.
WIP-MEI: patch converter.Converter.parseData() to detect MEI vs MusicXML