Add referred parts' hash strings in __buildout_signature__, that invokes rebuild of a part when one of its (recursive) dependencies are modified.

Also remove duplicates and sort entries in __buildout_signature__.

This patch give ability for Options instances to support set objects
as values.
It define __eq__ to avoid calling the fallback method defined in UserDict upper class
ie: UserDict.__cmp__
(cherry picked from commit 4ca599db4e4dfe7f830747dce89d066a0d5d468f)
Signed-off-by: Cédric de Saint Martin <cedric.dsm@tiolive.com>

Conflicts:
	CHANGES.SlapOS.txt
	setup.py

Make Options.copy return de-serialised values.
Make _save_option accept non-string values.

Useful when recipes generate non-string values to be reused by other
recipes.

Useful when recipes generate non-string values to be reused by other
recipes.

Conflicts:
	src/zc/buildout/tests.py

ref https://bugs.launchpad.net/zc.buildout/+bug/144228 .

This reverts commit dcea4db0.

Revert "incase of symlink, we should just delete it instead of calling os.chmod that tries to chmod the original that can raise 'Operation not permitted' error."

This reverts commit 2c2b8c43.

This reverts commit 0e9cb89e.