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
Kde sa da preferujeme google-style formatovanie (`Arguments:`) apod pred skaredym `:param`. Len niekedy to nepouzivam lebo blbne s * a ** argumentami.