Update list of known properties
Use valid XSD literal even for invalid time values
Fixed Turtle export of preferred calendar model
Use dump of current revisions instead of full dump

The smaller files are faster to download and process, and they
do not grow so strongly. They are now used in the export script
by default, which can be overwritten by the option --no-current.
Fixed file name for using current revision dumps
Gather more detailed statistics during export

A complete dump of the statistics is appended at the end of
the Turtle file in comments.
Updated known property list
Support selection of max date for historic exports

The optional command line parameter --max-date YYYYMMDD allows
one to create exports based on dumps up to a certain date, if
available (especially older dailies tend to vanish quickly).
Sanitize broken globe specifications
Merge branch 'master' of https://github.com/mkroetzsch/wda