Assert proper types in errors reported from checkers [#548]
Use identity as default filter for generic checkers [#548]

The default filter for command checkers is still
flycheck-sanitize-errors.
Don't expand nil properties
[Fix #548] Add flycheck-irony to documentation
Bump sphinxcontrib-emacs dependency for bug fixes
Show build date when running tests

Helps us to check whether Emacs snapshot is current.