PyPI (pip installation) setup for 0.6 code (#1445)

* force gcc-5 or clang-omp for Mac OS, prepare for pip pack

* add sklearn dep, make -j4

* finalize PyPI submission

* revert to Xcode clang for passing build #1468

* force to clang, try to solve cmake travis error

* remove sklearn dependency

python-package/prep_pip.sh

@@ -0,0 +1,11 @@
+# this script is for preparation for PyPI installation package, 
+# please don't use it for installing xgboost from github
+
+# after executing `make pippack`, cd xgboost-python,
+#run this script and get the sdist tar.gz in ./dist/
+sh ./xgboost/build-python.sh
+cp setup_pip.py setup.py
+python setup.py sdist
+
+#make sure you know what you gonna do, and uncomment the following line
+#python setup.py register upload