Squashed commit of the following:

commit c30e28424a07bb69db89d131a4668abfe59ada7b
Author: Tobias Stöckler <tobiasstoeckler@googlemail.com>
Date:   Sun Dec 4 11:32:35 2016 +0100

    Issue #2825940 Fix GuzzleDefinitionDiscovery

commit 2b9dc0938fed936a900d0a5bf2d14f58200b8dbc
Author: Tobias Stöckler <tst@bio.logis.de>
Date:   Fri Dec 2 18:12:36 2016 +0100

    Issue #2825940 fix remaining usages of YAML

commit eadad4ee31c7276ae9dfaa7a6f5d538674ddb1ed
Author: Tobias Stöckler <tst@bio.logis.de>
Date:   Fri Dec 2 17:11:59 2016 +0100

    Issue #2825940 Allow multiple remote definition registries

commit 410b1faca373b84ee0254f42083c6bad573e329a
Author: Tobias Stöckler <tst@bio.logis.de>
Date:   Fri Dec 2 17:11:26 2016 +0100

    Issue #2825940 Change default local serialization from YAML to JSON

commit 2af5198ada9f0a65eff5b2039bf689cabab4d7c5
Author: Tobias Stöckler <tobiasstoeckler@googlemail.com>
Date:   Sun Nov 13 12:55:32 2016 +0100

    Issue #2825940 by tstoeckler: Fix description of
    LibrariesConfigSubscriber::onConfigSave

commit 62d69f0b7aefa9d59c023f12725ceef1940fdb8b
Author: Tobias Stöckler <tobiasstoeckler@googlemail.com>
Date:   Sun Nov 13 11:20:23 2016 +0100

    Issue #2825940 bring back LibrariesConfigSubscriber to avoid test
    workaround

commit 5e120593bc514fdf1435408e165dc45977a3242b
Author: Tobias Stöckler <tobiasstoeckler@googlemail.com>
Date:   Sun Nov 13 11:18:02 2016 +0100

    Issue #2825940 by tstoeckler: Move LibrariesServiceProvider logic into a
    discovery factory