Fix: identifier values getters

`getIdentifierValues()` must always return an array and
`getNormalizedIdentifiers()` must return a string.

The `ModelManager` was updated according to these constraints and these
two methods now work with composite PK. The `find()` method was also
updated to work with composite PK.
Merge pull request #20 from K-Phoen/fix-identifiers-getters

Fix: identifier values getters