Update engine.js

Fixed <<print>> macro so that it doesn't print null or NaN, the previous logic allowed both.
Update header.html

Fixed <<choice>> so that it adds link text, previously it added none and thus was unclickable.
Update header.html

Fixed <<choice>> so that it adds link text, previously it added none and thus was unclickable.
Merge pull request #2 from tmedwards/master

Fixes for <<print>> and <<choice>>