prepare for 0.6

update oasis files for 0.5.2

update of readme and .ocamlinit

version 0.5, with updated README

version 0.4.1

version 0.4 (for the new API); more features should follow

updated META

version stored in datalog.logic

exception UnsafeClause is raised if an unsafe clause is added

pretty-print substitutions, and compare terms;

removed redundant datalog/logic.mli file (since datalog.mli subsumes it)