Feat/delete file model

[aitken85]

Updates FileModel->delete() to also delete the physical file

Tests fail!

Sprout\Exceptions\FileUploadException: Failed to move uploaded file
/srv/sprout3/src/sprout/Models/FileModel.php:214

This is the FileModel::fromPath unable to move file. So something setup wrong in my tests, not a fault in the delete method, hopefully.

This PR is mostly to get the ball rolling with any fixes that maybe required, then approved.

[gwillz]

<3 the unit tests

Hey, when we get nested transactions merged we can even do cool stuff like this:

$this->getConnection()->withTransaction(function() {
   $this->delete();
   File::delete($this->filename);
});

Meaning the file is only deleted if the DB commit works. Or if the DB commit fails, the file isn't deleted.