WIP: First attempt to approach the issue bob.bio.base#106

In this issue I'm:

1. Removing the loading and saving functions via HDF5 in the C++ and in the python API (not sure if it's really necessary to remove from the C++ level)

2. Implemented the methods to_dict and from_dict for the activation functions

3. Test cases

Maybe I would need a bob.nightlies just for this