.apply() API consistency fix

DataFrame.apply, Series.applymap, Series.map sort of come across as inconsistent.

```
def f(x):
    return x.apply(lambda y: 2*y)
def g(x):
    return x.applymap(lambda y: 2*y)

a = TimeSeries([1,2],[1,2])
b = DataMatrix({'a':a})

f(b)
g(b)
f(a)    
g(a)
```
