Introduced copyright notice in all source files.
Merged `oct.lib` into `oct.core`.

* Added copyright notice to test files as well.
Revised `core.Atom` to enhance maintainability.

* Streamlined exception use.
* Implemented `toString`, `hashCode` and `equals` for `common.Element`.
Revised the file `COPYRIGHT' for better clarity.
Minor revisions to `core.Bond` for correctness and better API.
Revised `core.Ring` for minor corrections.
Eclipse-based code clean-up.

* Three (trivial) accessors added to `core.Atom`.
* Re-formatted several `//`-style comments for maintainability.
Resolved issue #10.
Removed custom exception classes.

* Revised code using custom exception classes.
* `core.Molecule` attributes are now stored in two lists, to preserve
  input order.
Revised `core.Molecule` for correctness and maintainability.

* Propagated the changes to other classes, as needed.
Literature reference classes have been introduced.

* `core.Atom`, `core.Bond` and `core.Ring` have been made `final`.
* `core.Reaction` gets practically redefined; work in progress.
Functional groups and leaving groups appear.
Revised the possible states of an atom's unsaturation.