Infinity norm should take absolute value of elements

This fixes a small bug where
```
floats.Norm([]float64{1, -2}, math.Inf(1)) // => 1 (should be 2)
floats.Norm([]float64{-1, -2}, math.Inf(1)) // => -1 (should be 2)
```
See the wikipedia page for the definition. http://en.wikipedia.org/wiki/Norm_(mathematics)

TestNorm was modified to catch the problem.
Could you just put the function here instead of creating a new one, please?