Refactored the `ReflectiveMetaModelRegistry`.
Bugfixed to `FInternalObjectImpl#updateContainment`. Bugs were caused by
inverse adds on references with opposites. Don't know why the tests did
not cover that!
OPEN - task 20: "... has to be part of a fragmented model" 
http://github.com/markus1978/emf-fragments/issues/issue/20

Objects can get a preliminary extrinsic ID. That way they only cause
errors when the referencing object is to be persisted and the referenced
object is still not part of a fragmented model.
Added a test case for enums. See Issue: #21.
OPEN - task 21: Enum literals cause ClassCaseException 
http://github.com/markus1978/emf-fragments/issues/issue/21

Fixed this issue and adopted tests.
OPEN - task 22: Configuration of strictness
http://github.com/markus1978/emf-fragments/issues/issue/22

We have now the following configuration parameters:
- indexedValueSetBehavior = (strict|neverContains)
- extrinsicIdBahvaior = (strict|preliminary|defaultModel)

Added implementation and tests.
Minor bugfixes.
Bugfix: we now ensure to update the extrinsic id index, when an object
is saved. Before it happened, that `defaultModel` extrinsic id index
entries were empty.
Made mongodb plugin available, its activator now registeres mongodb as a
protocol/URI-scheme.
Bugfixes.