Add support for element IDs as path keys in get_path/2 and set_path/3

A path key can now be set to `{Key, ElementId}` and the corresponding
function will perform a lookup on a list of elements trying to match
the element whose `Key` has the value `ElementId`.
Add additional documentation and examples for get_path/2 and set_path/3

The new examples show the usage of path keys with element IDs
Update version to 0.4.0