Added field positioning functionality to FormMapper.

At the moment, in order to reorder fields, you have to pull out of a list of all fields, reorder them manually, and pass them back to the reorder method.

This is less than convenient, so I wrote a few basic move methods that wrap all of that up for you.

This adds support for moving fields to the first or last position, as well as before and after other fields. This is especially useful when extending other admins, and thus you can't simply specify the fields in the correct order in the first place.

Example usage:
```php
$formMapper
    ->add('amount')
    ->add('unit')
    ->add('name')

    ->move('amount', $formMapper::AFTER, 'name')
;
```
or
```php
$formMapper
    ->after('name')
        ->add('amount')
        ->add('unit')
        ->add('name')
    ->end()
    ->before('unit')
        ->add('another_field')
    ->end()
    ->with('Prices')
        ->first()
            ->add('and_another')
        ->end()
    ->end()
    // ..etc.
;
```
**Update**: Switched out magic strings for constants and added wrapping methods for adding fields at a particular location.