Those helpers cover most of slapgrid use cases: download AND upload from/to
string (fd), file, directory.

Fetch all trustable entries and let client decide which entries to use.

This reverts commit 2a9024fc.

This reverts commit 5ed93f4c.

Squashed commit of the following:

commit 5c74964a312fb743f88a115ffb4d5af44dd3d2d1
Author: Łukasz Nowak <luke@nexedi.com>
Date:   Fri Sep 2 10:22:48 2011 +0200

    No more M2Crypto is required.

commit 71ddc285850bd1c5761b95cff8616c6023f6eadf
Author: Łukasz Nowak <luke@nexedi.com>
Date:   Fri Sep 2 10:22:03 2011 +0200

    Drop M2Crypto dependency.

    Simplify, generate only common name in key.

commit 9e058070bc66fc91a590d90adf379a899b00d124
Author: Łukasz Nowak <luke@nexedi.com>
Date:   Thu Sep 1 18:05:14 2011 +0200

    Cover cases when openssl is not compatible.

    It seems that python executable can be seen as openssl begin not compatible.

commit 69901f159392db65597b97482439bea4d002e9c4
Author: Łukasz Nowak <luke@nexedi.com>
Date:   Thu Sep 1 18:00:27 2011 +0200

    Raise correctly in case of openssl unavailabilty during upload.

    More: raise DirectoryNotFound in more cases.

commit c17c46053385b586e7ccdb4ebf3ecb9ab46d901b
Author: Łukasz Nowak <luke@nexedi.com>
Date:   Thu Sep 1 18:00:08 2011 +0200

    Cover case when no openssl is available during upload.

commit d88d9e2b892000fa64c28b74182777fd03c93556
Author: Łukasz Nowak <luke@nexedi.com>
Date:   Thu Sep 1 17:55:23 2011 +0200

    Covert case when empty shadir entry is in shadir.

commit 6671546db794a5ea8e7025de115a613940e4af14
Author: Łukasz Nowak <luke@nexedi.com>
Date:   Thu Sep 1 17:45:08 2011 +0200

    Add no openssl during content selection test.

    Add another test skeleton.

commit ad6b3b5abbbb2d8267938661946acf1bd4d02b43
Author: Łukasz Nowak <luke@nexedi.com>
Date:   Thu Sep 1 17:32:09 2011 +0200

    Close another temporary file.

commit 34d15b585e52810908323cf846af5a64171f62b2
Author: Łukasz Nowak <luke@nexedi.com>
Date:   Thu Sep 1 17:23:23 2011 +0200

    Expose openssl and avoid dying in case if not exists.

commit 0eb456fc6ccd2d36644c47abc1ef79b7b51187c6
Author: Łukasz Nowak <luke@nexedi.com>
Date:   Thu Sep 1 17:15:54 2011 +0200

    Pep8isation.

commit 7179acf759a1d18bff367025a965d020443f93e2
Author: Łukasz Nowak <luke@nexedi.com>
Date:   Thu Sep 1 17:12:25 2011 +0200

    Drop M2Crypto dependency.

    M2Crypto is unmantained library. It seems safer to use openssl directly to
    sign and verify content.