Fix nested property configuration

Fixes #592
Move Indexable callback calculations to a new service
Implement callback checking in the provider
Update changelog
make sure headers is set prior to accessing
make PSR2 compliant
Merge pull request #609 from PeerJ/FixLoggerWithNullTransport

make sure headers is set prior to accessing
Merge pull request #593 from merk/gh-592

Fix nested property configuration
Added documentation changes for indexable_callback
Merge pull request #608 from merk/populate-indexable

Indexable callback refactoring, implemented callback in Provider
Fix incorrect provider configuration
Wrong service. Whoops.
Fix indexable callbacks with alias based indexes
Add unstable badge

[skip ci]
Update composer.json