Added fetching of time ranges (Obdobia).

* Using obdobie as an atomic unit of time range taken from AIS

* Added function get_obdobie which gets the time range based on its
  Slovak name which should be present in a listing in the VSST010
  application

* Added function get_semester_obdobie which returns the time range of a
  given semester, optionally in a given academic year

* Added function get_skuskove_obdobie which returns the time range of a
  given examination period

* Added Obdobie structure

* Closes #13