Allow import via python datatable. (#3272)

* Allow import via python datatable.

* Write unit tests

* Refactor dt API functions

* Refactor python code

* Lint fixes

* Address review comments

tests/travis/run_test.sh

@@ -48,6 +48,13 @@ if [ ${TASK} == "python_test" ]; then
     source activate python3
     python --version
     conda install numpy scipy pandas matplotlib nose scikit-learn
+
+    # Install data table from source
+    wget http://releases.llvm.org/5.0.2/clang+llvm-5.0.2-x86_64-linux-gnu-ubuntu-14.04.tar.xz
+    tar xf clang+llvm-5.0.2-x86_64-linux-gnu-ubuntu-14.04.tar.xz
+    export LLVM5=$(pwd)/clang+llvm-5.0.2-x86_64-linux-gnu-ubuntu-14.04
+    python -m pip install datatable --no-binary datatable
+
     python -m pip install graphviz pytest pytest-cov codecov
     python -m nose tests/python || exit -1
     py.test tests/python --cov=python-package/xgboost