Correcting MSVC warning C4800

Whenever you coerce a non-bool type to a bool, you get a 'performance warning'. This is because a simple truncation cannot be done when you coerce to bool; a test against zero has to be done first, and then the result used to set the boolean value either to "true" or "false".
Merge pull request #118 from leapmotion/bug-conversionwarning

Correcting MSVC warning C4800