properly escape all "invalid" characters in corpus names when creating empty corpus.properties

(fixes #310 #312)
updating version to 3.1.3 (next bugfix release)
fix some documentation issues (fixes #313)
make the SQL getter functions "STABLE" instead of "IMMUTABLE"
avoid joining the corpus table in "find"
Revert "make the SQL getter functions "STABLE" instead of "IMMUTABLE""

This reverts commit 6c7ebcfed18410f0c3f80773c33a71e635dfa93e.
workaround applied to frequency in order to fix #314
typo in documentation
updating changelog
Finish annis-3.1.3