Correct type mismatches in PromiseKit.m
Replace kCFNull with NULL

The `void *` parameter accepts `NULL`, so there is no reason to pass it
a CFNullRef that needs to be bridged.
Use a bridged NSNull instead of a kCFNull

Getting an error because kCFNull is declared as a const. Using a bridged
NSNull instance fullfills the same purpose, without a warning.
Merge pull request #12 from Abizern/minor-corrections

Minor corrections