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
Funguje mat medzeru medzi :class: a backtickom? Bol som v tom ze sphinx to chce mat pri sebe, ale v rST je to mozno jedno. Skus overit tym ze zmenis nieco v aisikl dokumentacii a buildnes.