Added 24 more tests to ring detection; 21 pass.

* Aromaticity of the ring system as a whole has been introduced.
* A few difficult cases need to be resolved.
* Planar nature of a ring vis-a-vis Huckel's rule needs to be
  understood better.
Added marking benzylic positions.
Renamed `lib.RingDetector` to `lib.DefaultRingDetector`.
Renamed `oct.core` to `oct.data`.
Renamed `oct.core` to `oct.data` in `test` directory.
Updated `README.md` to reflect the current status.