• Breaking changes: References have changed behaviour
• Reference doesn't extend Firebase anymore and has changed behaviour (see the [doc/02-references.md](documentation about References)).
• Reference data can be accessed with $reference->getData(), which returns an array, or directly with $reference['key'].
• Reference data can be updated with $reference->update(['key' => 'value']) or with $reference['key'] = 'value'.
• Improved test coverage.
• Added Makefile to ease test execution.
• Extended documentation and moved in to the doc folder.