Pivot NaN Bug
The 'pivot' function does not properly handle NaNs. E.g.:

```
import numpy as np, pandas as pd
test = pd.DataFrame({"a":['R1', 'R2', 'R3', 'R4'], "b":["C1", "C3", np.nan , "C4"], "c":[10, 15, np.nan , 20]})

test.head(2).pivot('a', 'b', 'c')
b   C1  C3
a
R1  10 NaN
R2 NaN  15
```

(No NaNs, works properly)

But:

```
test.pivot('a', 'b', 'c')
b   C1  C3  C4
a
R1 NaN NaN NaN
R2 NaN  10  15
R3 NaN NaN NaN
R4 NaN NaN  20
```

(Incorrect results, does not crash)

Bug is present in versions 0.8.1 and 0.10.1
