Add predmet_key to Termin struct.

Fixes #39.
Hm, toto riesenie sa PEP8 checkeru nepozdava (`E126 continuation line over-indented for hanging indent`).

Takto je to podla PEP8 ok, moze by?
```
key_defs = [_keyed_namedtuple_key_template.format(
    key_name=key_name, key_args=', '.join(key_args))
    for key_name, key_args in key_fields.items()]
```