Adding @ExposedField annotation

[moufmouf]

The @ExposedField can be used to quickly expose a field in the underlying object:

/**
 * @Type(class=Post::class)
 * @ExposedField(name="name")
 */
class PostType extends AbstractAnnotatedObjectType
{
}

[coveralls]

Coverage increased (+2.5%) to 73.123% when pulling fb7152f on moufmouf:exposedfield into b20fc0e on thecodingmachine:master.