Repackage/rename core and storage classes (#391).

- Rename "MehtaGraph" interface to "DeepaMehtaStorage".
- Rename "MGStorageBridge" class to "StorageDecorator".

- Combine packages de.deepamehta.core.impl.[service|storage] just into "de.deepamehta.core.impl"

Neo4j storage implementation:

- Rename "dm4-mehtagraph-neo4j"           module to   "dm4-storage-neo4j"
- Rename "deepamehta-mehtagraph-neo4j"    artifact to "deepamehta-storage-neo4j"
- Rename "de.deepamehta.mehtagraph.neo4j" package to  "de.deepamehta.storage.neo4j"
- Rename "Neo4jMehtaGraph"                class to    "Neo4jStorage"

See ticket 391.