Migration generation

This is my first pass at making migration baking a little easier to use.

- Moved the existing bake behavior to a `--snapshot` option
- Moved the template to `migration.ctp` to `snapshot.ctp`
- Cleaned up the actual bake task a bit
- Added a `MigrationHelper` class to handle data output for migration files

This PR handles the signed/unsigned bug, and adds support for more phinx-supported attributes.

TODO:

- [x] Update docs
- [x] Rails-style migration generation
- [ ] Test bake output the same way that the core does
- [ ] Add tests for the `MigrationHelper` class
Actually it should be

```php
$arr = preg_split('/(?=[A-Z])/', $className);
unset($arr[0]); // remove the first element ('')
$fileName = date('YmdHis') . '_' . strtolower(implode($arr, '_')) . '.php';
```