Bit count

I'd like to use an integer column (16 bits will suffice) to hold a bit-field. I'd like to be able to efficiently count
thenumber of bits set in this field. 

Is there a built-in function call I can use? (I can't find one in the manual).

If not, can anyone recommend the most efficient way within postgres to implement the kind of bit-counting tricks found
at:

http://gurmeetsingh.wordpress.com/2008/08/05/fast-bit-counting-routines/
http://graphics.stanford.edu/~seander/bithacks.html

Nathaniel