Serialization (#75): add PropertyContainer::toJSON().

Along with a GraphDatabase helper to get the JSON foundation.
Serialization (#75): add GraphDatabase::fromJSON().

Along with a static PropertyContainer.fromJSON() to do the actual parsing.
Serialization (#75): add test cases.
Serialization (#75): add GraphDatabase::reviveJSON() method.

For direct pluggability into JSON.parse()!
Serialization (#75): update changelog.
Merge branch 'feature/serialization'