Simple sending ether to contract address works the same as calling buyWithLimit() with zero limit.
But what is really needed and NOT explained in your article - how to claim tokens and how to generate and register EOS key (without which yoo'll be unable to use tokens after EOS blockchain launch).

Also, if you contribute to a closed window this instruction in buyWithLimit()
assert(day >= today());
will throw exception. Transaction'll fail using all gas.