Add LocalFile field type to allow uploading files to the local filesystem

This is essentially the `s3file` field except instead of uploading to amazon, it moves the uploaded file to the `dest` directory you specify in the field options. Instead of an `upload` hook there is a pre and post `move` hook which allows you to do stuff before and after the file is moved to the destination directory.

As a side note, the field doesn't delete the uploaded file when the instance is deleted from the database. This might be a bug in keystone, whereby the field doesn't get a chance to handle the delete as the request handler is not invoked. It seems keystone just deletes the thing from the DB without consulting it - this is out of scope so not fixed here.