[SPARK-2190][SQL] Specialized ColumnType for Timestamp

JIRA issue: [SPARK-2190](https://issues.apache.org/jira/browse/SPARK-2190)

Added specialized in-memory column type for `Timestamp`. Whitelisted all timestamp related Hive tests except `timestamp_udf`, which is timezone sensitive.
I also came to this solution at first, but I'm not sure whether changing application wide timezone settings causes any trouble somewhere else, an example is log timestamp. I'll have a try later.