drop column helper support arrays

Support arrays similar to alterColumn

example:

```
var query = {
  type: 'alter-table'
, table: 'users'
, action: {
    dropColumn: [
      { name: 'phone' }
    , { name: 'email' }
    , { name: 'twitter' }
    ]
  }
};
``` 

```
alter table user
  drop column phone,
  drop column email,
  drop column twitter;
```