Enable file checker to handle remote storage.
Enable file checker to handle remote storage.

Make FileChangedChecker more flexible: currently assumes file_stat is
provided by os.stat which works only for local files. Make this
customizable by adding two methods
- exists: used by Dependency class to check if target(s) exist. Defaults
to os.path.exists
- info: used by Dependency class to compute the file's stats. Defaults
to os.stat

With the addition of these two methods it's easy to handle AWS S3 and
other cloud storage.

pydoit/doit