Add fetching study schedules.

update: Using Obdobia for time ranges

update: Updated terminy, using obdobia instead

* 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

update: Addressed @TomiBelan's comments

* Fixed whitespace all around.
* Using code of the time range as a selector.
* Added docstrings where appropriate.

update: Adressed even more @TomiBelan's comments

* Fixed various issues w.r.t. docstrings (''' on a separate line,
  grammar fixes, ...)

* Using google-style docstrings where possible.

update: Reverted misunderstood comments

* Reverted removal of akademicky_rok