ENYO-2949: better specify `overwrite: undefined/true/false`

* Tested on OSX/ChromeCanary
* **DO NOT MERGE** before 2013-09-05

- `overwrite != false` means `overwrite = true`
- add file-system test cases
- set file-manager operations create/copy/move to `overwrite=false`

Enyo-DCO-1.1-Signed-off-by: Francois-Xavier KOWALSKI <francois-xavier.kowalski@hp.com>

This is an iteration on top of https://github.com/enyojs/ares-project/pull/504, already accepted & then reverted.