timestamp precision added

Added explicitly precision to timestamp, to remove unnecessary precision. I dont think that someone need extra precision, it is not a common use-case. 
Also it is more useful and handful to work on php side with such precision. If someone still need extra precision he can manually create it in migration or in query builder by specifying explicitly column string.

```sql
CREATE TABLE testing_table (
 id serial NOT NULL,
 description character varying(255),
 time_without_precision_tz timestamp(0) with time zone NOT NULL DEFAULT now(),
 time_without_precision timestamp(0) without time zone NOT NULL DEFAULT now(),
 time_with_precision_tz timestamp with time zone NOT NULL DEFAULT now(),
 time_with_precision timestamp without time zone NOT NULL DEFAULT now(),
 CONSTRAINT testing_table_pk PRIMARY KEY (id)
)
```

```sql
INSERT INTO testing_table(description)
    VALUES ('some_description_0');
INSERT INTO testing_table(description)
    VALUES ('some_description_1');
```

```yaml
SELECT * FROM testing_table;
-[ RECORD 1 ]-------------+------------------------------
id                        | 1
description               | some_description_0
time_without_precision_tz | 2014-05-14 17:42:42+04
time_without_precision    | 2014-05-14 17:42:42
time_with_precision_tz    | 2014-05-14 17:42:41.662101+04
time_with_precision       | 2014-05-14 17:42:41.662101
-[ RECORD 2 ]-------------+------------------------------
id                        | 2
description               | some_description_1
time_without_precision_tz | 2014-05-14 17:42:42+04
time_without_precision    | 2014-05-14 17:42:42
time_with_precision_tz    | 2014-05-14 17:42:41.662101+04
time_with_precision       | 2014-05-14 17:42:41.662101
```