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
Split off find_row and find_option.
Encapsulate the keys of fladgejt.structures.

We refer to most fladgejt structures by some subset of their fields that we
assume will be immutable and mostly unique. These are used as a key in
find_row(), and later request_json(), to look up the full instance. But most
fladgejt methods don't care about the key's exact composition. Instead of
having the key's fields directly in argument lists, we group them in a tuple.
Converting a full instance to this smaller tuple can be done with a new "key"
field that is automatically initialized on namedtuple creation.

Fixes #12.
Add confim box support.

enable to emulate clicking on buttons when a confirm box is showed
Add support for adding and deleting zapisny list.