"checker" is the name being used everywhere.

This sucks, but otherwise I get occasional Fs on Mac OS X.

Mac OS X tries by all means to hide that /var is /private/var, and that is
what FSEvents reports back.

In particular files are no longer created in the current working directory,
but in a temporary folder.

This commit also bases everything on Pathname internally.

3.0.3 has a bug in OS X.

This is the implementation of the file update checker written
by Puneet Agarwal for GSoC 2015 (except for the tiny version
of the listen gem, which was 3.0.2 in the original patch).

Puneet's branch became too out of sync with upstream. This is
the final work in one single clean commit.

Credit goes in the first line using a convention understood
by the contrib app.

fix method name typo [ci skip]

ActiveJob docs text fix. [ci skip]