Clip Could take Series/DataFrames
 I was a little surprised when clip would not take other pandas objects as parameters.  I hacked a little something below, but for readability I thought I might suggest that this feature be added.

```
df1 = DataFrame(ones((2, 2)))
data = ones((2, 2))
data[0, 0] = 0
data[1, 1] = 2
df2 = DataFrame(data)
print df2.clip(1.5, 0.5)

df3 = df2.copy()
df3[df2 > df1 * 1.5] = (df1 * 1.5)[df2 > df1 * 1.5]
df3[df2 < df1 * .5] = (df1 * .5)[df2 < df1 * .5]
print df3
```
