TODO

* Fix::Condition documentation
* (better) error handling
* logger
* add catmandu-examples as tests
* validation
* remove Fixable, add before_add and before_get, takes fixes or sub
* give Iterable fix method
* give Iterable export, export_to_string methods
* bag drop
* Searchable handle index optimize
* split Searchable into Searchable and CQL Searchable (?)
* generator should guard with a magic ref, not undef (?)
* make serializers pluggable and split off MessagePack serializer

