Fix setDebugMode: call in Obj-C plugin API

This is intended to fix the oddity where setDebugMode: receives an NSNumber instead of a BOOL in spite of the plugin interface (in Obj-C) specifying a BOOL, resulting in pretty much all plugin code using setDebugMode being up to a coin-toss's chance as to whether debug mode is actually enabled (since it just results in a pointer being truncated to a bool, which could be either true or false in the end).

Also corrects some typos.

Should ideally close #36.