remove HDF5able making room for h5it

`hdf5able` has no morphed into [`h5it`](https://github.com/menpo/h5it), a more ambitious generic pickle replacement. In response to this, we:

1. No longer need `HDF5able` subclassing in Menpo
2. No longer need to depend on hdf5able/h5it (we'll strongly promote it, but it shouldn't be forced on users of menpo)
3. No longer need menpo.io.{save,load} (just pickle/h5it)

This PR removes all traces of hdf5able. A future PR will add back pickle specialisation from `serializablecallable`, once that has stabilised a little.