Add logical or

[yegct]

Two fixes:

• match result to return value of function; this change does affect resulting executable
• add missing EXPRESSION_LOGICAL_OR; I think this change does not affect resulting executable

Feel free to simply close this pull request if you don't want it, I won't take any offense.

[nibblebits]

Hi,
The request will be kept open for people who might be interested to use it, I cannot merge it with the main repository because it has to follow the lecture numbers so that students can easily see which commit belongs to which lecture.

Thanks for bringing this to my attention

Dan