Add comments, fix some issues with the model

* add ObjectMetadata::$title
* add Source::$contextUri
* add UsageRestriction::$type
* remove Work::$description
* change Work::$type into Work::$types (multivalue)
add todo list
Correct types

Switch 'type' variables to string, texts to mono- or multilingual
text.
Reorder functions

PHPStorm has the annoying habit of generating getters/setters in
completely random order; reorder them to match the properties.
(+3 OCD points)
Introducing transformation logic.

This introduces interfaces and some experimental implementations
for import and export transformers.
Top level import test script
Merge branch 'IntroducingTransformers' of github.com:creative-work-metadata/creative-work-metadata into IntroducingTransformers