Added missing data type “TIMESTAMP”

**Problem**
It doesn't allow me to create a table that contains the `TIMESTAMP` type. It always returns <code>BigQuery::Errors::BigQueryError: Invalid value for: null is not a valid value</code>.

**Solution**
Missing data type “TIMESTAMP” has been added. See [possible data types](http://cloud.google.com/bigquery/preparing-data-for-bigquery#datatypes).