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.

Squashed commit of the following:

commit 2c3b7f2f
Author: Łukasz Nowak <luke@nexedi.com>
Date:   Wed Aug 31 16:36:18 2011 +0200

    Prove that keys are passed to libnetworkcache.

commit c380b1ea
Author: Łukasz Nowak <luke@nexedi.com>
Date:   Wed Aug 31 15:42:05 2011 +0200

    Support shacache and dir SSL based auth.

Thanks to (un)patching urllib before importing zc.buildout.download no more
M2Crypto cluttering is done.

This allows complex library usage in bootstrap process.

Certificates to check content are provided as inline strings.

also convert those certificates correctly.

Convert buildout emptiness to Nones.

Certificates to check content are provided inline.

Use signature_certificate_list if strings instead of signature_certificate_file.
Additionaly simplify backward compatbility: just do not play with it.

Use simple download recipe instead of relying on egg downloading.

In addition comment why it does not work.

Instead use provided system level tools to generate keys.

This reverts commit bfe5fb83.