Use net_gearman instead of pecl/gearman, fixes #4767

This commit replaces sfGearmanPlugin with a simpler daemon task based in
net_gearman (pure PHP) instad of pecl/gearman, wich was being a source
of problem between our users for different reasons, e.g. some incompabilities
between pecl/gearman, libgearman and gearmand causing errors in the worker.

Squashed commit of the following:

commit f020112dc7fe88502e22da2f3ffc9604b8fce46c
Author: Jesús García Crespo <jesus@sevein.com>
Date:   Fri Mar 8 08:34:43 2013 -0800

    Use the event dispatcher to log events

commit 4f3242adbeb3a9fbc63c5fdd52e4e4bb69a28b7b
Author: Jesús García Crespo <jesus@sevein.com>
Date:   Thu Mar 7 21:29:25 2013 -0800

    Adapt worker to be compatible with net_gearman

commit 199163bae609d296100fda9eebb4cf4729a046d3
Author: Jesús García Crespo <jesus@sevein.com>
Date:   Thu Mar 7 21:28:55 2013 -0800

    Add simple gearman daemon task

commit be6a28b4d19cda25f6dfdddc7c84d7daf2d577a1
Author: Jesús García Crespo <jesus@sevein.com>
Date:   Thu Mar 7 21:28:37 2013 -0800

    Autoload net_gearman recursively

commit d4c8e055aa6cdfd68d363dfe01dff5fad4a8e2f7
Author: Jesús García Crespo <jesus@sevein.com>
Date:   Thu Mar 7 21:28:02 2013 -0800

    Update gearman.yml schema

commit dade534f7be5d01534991ad453a9db350ce3933c
Author: Jesús García Crespo <jesus@sevein.com>
Date:   Thu Mar 7 21:27:47 2013 -0800

    Update config to not load sfGearmanPlugin

commit 2d3630a5164a9fdfc1b2c56262759622f1d9eb20
Author: Jesús García Crespo <jesus@sevein.com>
Date:   Thu Mar 7 21:27:00 2013 -0800

    Update init script

commit f2689d399b2a17893658c9d6b10ccc52574753a7
Author: Jesús García Crespo <jesus@sevein.com>
Date:   Thu Mar 7 21:26:48 2013 -0800

    Remove sfGearmanPlugin, based in pecl/gearman

commit abcdf72981e71f81b1a9bfb32885eaa85c33ad54
Author: Jesús García Crespo <jesus@sevein.com>
Date:   Thu Mar 7 21:26:24 2013 -0800

    Add net_gearman