Squashed commit of the following:

commit 2d7dc0b3da46d41b8f3754a301d5662ed2bb64d5
Author: Łukasz Nowak <luke@nexedi.com>
Date:   Tue Sep 6 17:22:13 2011 +0200

    Explain more about patches.

commit d5df4cdcf915f31d35a0aea8cfa465c66e94d881
Author: Łukasz Nowak <luke@nexedi.com>
Date:   Tue Sep 6 17:18:55 2011 +0200

    Provide better looking patches.

commit c6befb0568e21d95ee2b9e96a0490dadf96b7c06
Author: Łukasz Nowak <luke@nexedi.com>
Date:   Tue Sep 6 17:15:32 2011 +0200

    Support make-command and make-options.

    Fix configure-command.

commit d2212cb7fa4e7d31af671b33b32fdf97094f7e6e
Author: Łukasz Nowak <luke@nexedi.com>
Date:   Tue Sep 6 17:04:05 2011 +0200

    Allow to override configure command.

commit 4321b38a15de7e5f564656b44acf2c5b84b79eee
Author: Łukasz Nowak <luke@nexedi.com>
Date:   Tue Sep 6 16:56:49 2011 +0200

    Share applyPatchList with in base class.

commit 4a2ea3b376546df7ddf4d08b58b9b8651be4c324
Author: Łukasz Nowak <luke@nexedi.com>
Date:   Tue Sep 6 16:56:29 2011 +0200

    Support no md5sum.

commit 2cd36cb7e07b80bbcb1bdbe29108e49899d9ad05
Author: Łukasz Nowak <luke@nexedi.com>
Date:   Tue Sep 6 16:56:16 2011 +0200

    Show how to patch.

commit 8cd9ea9c02fea1e81b435b5ad2401ead2898330d
Author: Łukasz Nowak <luke@nexedi.com>
Date:   Tue Sep 6 16:53:45 2011 +0200

    Allow options per patch.

commit 84fea6230aeb950cf683e9dc1665fdb74f8e1fd9
Author: Łukasz Nowak <luke@nexedi.com>
Date:   Tue Sep 6 16:48:47 2011 +0200

    Allow patching.

commit 8934eb92f100e5f7916bdea0bffd65287cee7558
Author: Łukasz Nowak <luke@nexedi.com>
Date:   Tue Sep 6 16:30:46 2011 +0200

    Allow to keep directories in case of error.

    Default is to remove.

commit 74d0b2bd41a827646a1a1fb851f0dbe8185a4965
Author: Łukasz Nowak <luke@nexedi.com>
Date:   Tue Sep 6 16:24:49 2011 +0200

    Allow to have a=b=c, where a is key and b=c is value.

commit 034c238125fd040c39c87afce6ea4aa6150ed059
Author: Łukasz Nowak <luke@nexedi.com>
Date:   Tue Sep 6 16:17:07 2011 +0200

    Another typo.

commit f5abcde019f257303f3e86b45792e98ba5dde010
Author: Łukasz Nowak <luke@nexedi.com>
Date:   Tue Sep 6 16:15:44 2011 +0200

    Follow fixed typo.

commit 5b950e2619beda41904f681b96a6ec05eeec3eab
Author: Łukasz Nowak <luke@nexedi.com>
Date:   Tue Sep 6 16:14:35 2011 +0200

    Extract does not need md5sum.

commit 6f725632c3574eb3b0d03493b166b385c862adb0
Author: Łukasz Nowak <luke@nexedi.com>
Date:   Tue Sep 6 16:14:14 2011 +0200

    Pass md5sum directly.

commit 9f42d22a38db59a1e3527e19b82af4886657ff52
Author: Łukasz Nowak <luke@nexedi.com>
Date:   Tue Sep 6 16:13:53 2011 +0200

    Allow to pass empty md5sum.

commit c91fafb1736406cbcb32b36af99277a46b67372d
Author: Łukasz Nowak <luke@nexedi.com>
Date:   Tue Sep 6 16:13:15 2011 +0200

    Fix typos.

commit 8919378ddc44169af07426a5cce6aa55b064f9c9
Author: Łukasz Nowak <luke@nexedi.com>
Date:   Tue Sep 6 16:12:31 2011 +0200

    Fix examples.

slapos.cookbook provided it before, but it is simple functionality, which does
not need so many dependencies.

Additionaly support slapos_update_promise

pipeCommand allows to simulate shell pipes.

failIfPathExists allows to stop in case of path which exists.

Read md5sum passed to method directly.

This reverts commit 94b519d8.

This reverts commit 667ec876.