g0v-data/president#3: Export as iCal format

For example, run as following:

  $ virtualenv.py runtime
  $ source runtime/bin/activate
  $ pip install icalendar
  $ python json2ics.py president.json
  $ deactivate
Rename method json2ical to json2ics.
Merge pull request #4 from jeffhung/master

g0v-data/president#3: Export as iCal format