Currently `pip install xgboost` will raise traceback like this
```
Traceback (most recent call last):
File "<string>", line 20, in <module>
File "/tmp/pip-build-IAdqYE/xgboost/setup.py", line 20, in <module>
import xgboost
File "./xgboost/__init__.py", line 8, in <module>
from .core import DMatrix, Booster
File "./xgboost/core.py", line 12, in <module>
import numpy as np
ImportError: No module named numpy
```
We should avoid importing numpy in setup.py and let pip install numpy and scipy automatically.
That's what `install_requires` for.
