Introduced generic classes for the 'registry-provider' pattern.

Signed-off-by: Dennis Nobel <d.nobel@external.telekom.de>
Refactored ThingRegistry, ItemRegistry and ItemChannelLinkRegistry to use generic classes and adapted according tests and dependencies.

Signed-off-by: Dennis Nobel <d.nobel@external.telekom.de>
Merge pull request #54 from qivicon/generic-provider

Introduced generic classes for the 'registry-provider' pattern and adapted according implementations to use new classes.