Add ensureDir; make mkDir complain when directory exists

Add IOEntryExists for the case when an object exists already.

Improve the FIXME about the trailing slash issue for pathExists, and fix
some other comment typos.

Remove some specification comments in staticNodeJS.js which duplicate
those in _base.js.

Add a warning about misuse of pathExists to _base.js.
Merge pull request #36 from rrthomas/master

Add ensureDir; make mkDir complain when directory exists