Add a Phinx\Db\Table::addTimestamps() method.

This method automatically create two timestamp columns: created_at and updated_at.

Using:

```php
// ...
$table->addTimestamp();
// ...
```
your table has two more fields: `created_at` and `updated_at`, both `timestamp`.
The `create_at` field IS NOT NULL and `updated_at` accept NULL with defaults NULL.


I use `timestamp` columns to allow #68 approach like @valorin said.

@valorin I think using a decent ORM this fields can be updated using callbacks. Isn't excess code.
The updated_at field should be empty after insert statement.

@czogori I think it is a good approach the columns names are not configurable, prefer convention over configuration.
If you need different column name, you can easily add manually.

Closes #41