Fix #808; Invalidate cache upon change of settings

The infrastructure has been added to invalidate the
file cache if the settings of a project descriptor have
changed and this has been verified with the Parse private functionality.
Fix #813; Verbose mode didn't work

The threshold when to output logs or not did a string comparison but
this does not work with the new PSR-3 LogLevels. As such a conversion
needed to be introduced that converts the PSR-3 Loglevels to a numeric
equivalent so that a correct comparison could be made.
Add test for Compiler class