Replaced own bool typedef with <stdbool.h> where available

This shouldn't break anything, but I'd like some feedback before merging it.

gcc and clang support C99, and VS2013 will include stdbool.h, according to http://blogs.msdn.com/b/vcblog/archive/2013/07/19/c99-library-support-in-visual-studio-2013.aspx