update: Added study dates API

* Created WebuiStudijneTerminyMixin which provides get_terminy_semestra,
  get_terminy_zimny_semester and get_terminy_letny_semester.

* Added TerminySemestra structure.

* Fixes https://github.com/fmfi-svt/votr/issues/7
Zabudol som sa spytat v commente, aky vlastne docstring sa ide pouzivat. Kludne mozeme google-style, len v niektorych suboroch som tu vidiel aj tie param, tak som vazne nevedel.