Remove Epoch, Event, and Spike objects

The first major backwards-incompatible change for neo.  This pull request removes the existing scalar types, ``Epoch``, ``Event``, and ``Spike``.  It also renames ``EpochArray`` to ``Epoch` and ``EventArray`` to ``Event``.

All ``neo.core`` unit tests pass.  However, as discussed on the mailing list, ``neo.io`` maintainers will need to check for failures in their classes and create pull requests for fixes.  The unit tests will be in a failing state until then.