Support smallserial and bigserial data type.

Unlike mysql's `auto_increment` constraint, postgresql has 3 different types for autoincrement field `smallserial`, `serial`, `bigserial`.

For example,

```
db.createTable('user', {
id: { type: type.BIG_INTEGER, primaryKey: true, notNull: true, autoIncrement: true }
})
```

I tried this code but it creates `serial` type which is unexpected behavior. From quick look of source code, I think it would be same for `smallserial` type as well. It could be easily fixed like below.

``` js
createColumnConstraint: function(spec, options, tableName, columnName) {
var constraint = [],
cb;

if (spec.primaryKey && options.emitPrimaryKey) {
if (spec.autoIncrement) {
switch (spec.type) {
case type.SMALL_INTEGER: constraint.push('SMALLSERIAL');
case type.BIG_INTEGER: constraint.push('BIGSERIAL');
default: constraint.push('SERIAL');
}
}
constraint.push('PRIMARY KEY');
}
...
```
##

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