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
Hej, vsade kde sa len da sa snazim pouzivat google style. Tento defaultny sphinxovy mi prisiel prilis skaredy (aspon v nejakych ohladoch, napr ked parametrom davam typ), a numpy style je zase prilis roztahany. Este keby fungovali aj tie hviezdicky...