Current Version doesn't build with dev numpy
Can't build current pandas with dev version of numpy. I get the following error:

```
In file included from pandas/src/tseries.c:258:0: pandas/src/skiplist.h:31:28: error: redefinition of ‘__npy_nanf' /usr/local/lib/python2.7/site-packages/numpy/core/include/numpy/npy_math.h:19:25: note: previous definition of ‘__npy_nanf’ was here
```

This seems odd as a previous commit removed
    #include <numpy/npy_math.h>

Commenting the offending function in `skiplist.h` seems to work, but that can't be the correct solution.
