Fixes kew fcall() behavior and adds nfcall()...

..., which is what the README as incorrectly claiming fcall() did before.

- The intention of fcall() is to delay invocation of a function, not
just call it right away.
- Q's nfcall() is the shortcut for getting promises back from functions
that use node-style callbacks. Adds that function to kew.
- Implemented nfcall() in terms of bindPromise(), since it does almost
the exact same thing.
- Fixes README documentation for fcall() and adds docs for nfcall().

Note: Looks like some kew functions use setTimeout() while others use
process.nextTick(). I used process.nextTick() here for fcall().