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
No, ja som myslel az do dneska, ze sphinx ine nevie precitat a preto som realne nemal na vyber. Situacia sa ocividne zlepsila.