This builds on https://github.com/apache/spark/pull/5932 and should close https://github.com/apache/spark/pull/5932 as well.

As an example:
```python
df.select(when(df['age'] == 2, 3).otherwise(4).alias("age")).collect()
```

Author: Reynold Xin <rxin@databricks.com>
Author: kaka1992 <kaka_1992@163.com>

Closes #6072 from rxin/when-expr and squashes the following commits:

8f49201 [Reynold Xin] Throw exception if otherwise is applied twice.
0455eda [Reynold Xin] Reset run-tests.
bfb9d9f [Reynold Xin] Updated documentation and test cases.
762f6a5 [Reynold Xin] Merge pull request #5932 from kaka1992/IFCASE
95724c6 [kaka1992] Update
8218d0a [kaka1992] Update
801009e [kaka1992] Update
76d6346 [kaka1992] [SPARK-7321][SQL] Add Column expression for conditional statements (if, case)