According to specification networkcache usage in buildout shall be fully
optional. The missing part was to make bindings immune on slapos.libnetworkcache
API changes or its internal errors.

Squashed commit of the following:

commit 79064a17
Author: Łukasz Nowak <luke@nexedi.com>
Date:   Thu Sep 22 13:12:54 2011 +0200

    Proof that bindings to nc are immutable.

commit b9795d85
Author: Łukasz Nowak <luke@nexedi.com>
Date:   Thu Sep 22 11:35:38 2011 +0200

    Immune buildout on any networkcache issues.

    As fallback shall be always possible make binding really immune to any problems
    in networkcache including name errors, api changes, syntax errors, etc.

commit ff3a32b9
Author: Łukasz Nowak <luke@nexedi.com>
Date:   Thu Sep 22 11:22:10 2011 +0200

    Show how networkcache is installed.

commit eeaccfc2
Author: Łukasz Nowak <luke@nexedi.com>
Date:   Fri Sep 2 11:14:49 2011 +0200

    M2Crypto is not needed anymore.