global: allow skip single elements

[david-caro]

• Add the possibility to skip single elements when using the for_each decorator, either raising the IgnoreElem exception or using the ignore_list_elem decorator.

Signed-off-by: David Caro david@dcaro.es

[jirikuncar]

• [x] Needs tests.
• [x] s/Add the possibility/Adds the possibility
• [x] Add yourself to https://github.com/inveniosoftware/dojson/blob/master/AUTHORS.rst
• [ ] This branch has conflicts that must be resolved.

[jirikuncar]

closed via #175