Add documentation to ws_value_union_reinit()
Add missing memberof to documentation of ws_value_union_init_from_val()
Merge pull request #452 from neithernut/fix-value-union-documentation

Fix value union documentation
Add method for ws_object type for checking whether a command callback is available
Add implementation for ws_object_has_cmd()
Add command for checking whether an object has a method
Add ws_object method for checking whether an object has an attribute
Add implementation for ws_object_has_attr()
Add command for checking whether a object has an attribute