can set primary keys for list nodes
more test on primary keys
get_verbose_name()
bugfix: primary keys were not parsed below root level
numerical names are prepended by _, 12 becomes _12
node trueism
if 'child' in parent is true
do not use parent in __init__
Squashed commit of the following:

commit 1f930a7d95f8079380d01db7153d968f4f1fa859
Author: Miklós Koren <miklos.koren@gmail.com>
Date:   Sun Mar 24 17:00:34 2013 +0100

    rename to DataTree() and change interface

commit 3f84073798c28d57e56542d5dc32119302920ebd
Author: Miklós Koren <miklos.koren@gmail.com>
Date:   Sun Mar 24 16:54:53 2013 +0100

    _deserialize takes file, not its content. CSVReader()

commit 71feeee88a4dd8d1b4830d37956d76e7b23a2aca
Author: Miklós Koren <miklos.koren@gmail.com>
Date:   Sun Mar 24 15:31:14 2013 +0100

    JSONReader class

commit 6095154cf00dafed4329093f7c6ffc4629afa3c6
Author: Miklós Koren <miklos.koren@gmail.com>
Date:   Sun Mar 24 15:26:06 2013 +0100

    refactor YAML reading into YAMLReader() and FolderReader()

commit 7ad810e2c2536e57bc5040c059b693265e05034a
Merge: 31f8693 b67407b
Author: Miklós Koren <miklos.koren@gmail.com>
Date:   Fri Mar 15 12:58:15 2013 +0100

    Merge branch 'master' into datatree

commit 31f8693c1efbf2ab89d48a873510e83f303c657e
Author: Miklós Koren <miklos.koren@gmail.com>
Date:   Fri Mar 15 12:54:35 2013 +0100

    new container classes

commit bc150db48d298233c1c0cdbd2b7c8bd8567c7a8a
Author: Miklós Koren <miklos.koren@gmail.com>
Date:   Fri Mar 15 12:46:56 2013 +0100

    datatree will read more than YAML files
empty strings are false
executables