Make python package wheelable (#1500)
Make python package wheelable (#1500)

Currently xgboost can only be installed by running:

    python setup.py install

Now it can be packaged (in binary form) as a wheel and installed like:

    pip install xgboost-0.6-py2-none-any.whl

distutils and wheel install `data_files` differently than setuptools.
setuptools will install the `data_files` in the package directory whereas the
others install it in `sys.prefix`. By adding `sys.prefix` to the list of
directories to check for the shared library, xgboost can now be distributed as
a wheel.
dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost