Rename Everything

### Classes
`League\FactoryMuffin\FactoryMuffin` => `League\FactoryMuffin\Factory`
`League\FactoryMuffin\Facade\FactoryMuffin` => `League\FactoryMuffin\Facade`

### Namespaces
`League\FactoryMuffin\Exception` => `League\FactoryMuffin\Exceptions`
`League\FactoryMuffin\Generator` => `League\FactoryMuffin\Generators`

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This pull extends #185 and #188.