Author: Reynold Xin <rxin@databricks.com>

Closes #5802 from rxin/SPARK-7274 and squashes the following commits:

19aecaa [Reynold Xin] Fixed unicode tests.
bfc1538 [Reynold Xin] Export all Python functions.
2517b8c [Reynold Xin] Code review.
23da335 [Reynold Xin] Fixed Python bug.
132002e [Reynold Xin] Fixed tests.
56fce26 [Reynold Xin] Added Python support.
b0d591a [Reynold Xin] Fixed debug error.
86926a6 [Reynold Xin] Added test suite.
7dbb9ab [Reynold Xin] Ok one more.
470e2f5 [Reynold Xin] One more MLlib ...
e2d14f0 [Reynold Xin] [SPARK-7274][SQL] Create Column expression for array/struct creation.