[WIP] Release 1.0

This is a _Work in Progress_ pull request for collecting all changes related to the 1.0 release candidate. The following things should either be discussed and rejected or implemented:

* [x] Remove `fineuploader` as default value for the frontend configuration key.
* [x] Make request passing in ValidationEvent mandatory. (#23)
* [x] Fix [Scrutinizer errors](https://scrutinizer-ci.com/g/1up-lab/OneupUploaderBundle/inspections/71d2d639-a07a-4353-beb4-9a4f8060a4e9/issues/?status=existing) and notices.
* [x] Make the temporary chunk directory configurable, so one could use Gaufrette. This is important if you want to use chunked uploads behind a load balancer. (#53)
* [x] It should be configurable to store uploaded files asynchronously. This could be a performance improvement if your application deals with big files. (#58)
* [x] Add a _Next step_ section to the frontend documentation files.
* [x] Remove extension validators. They fit no needs and are a potential security hole. (#59)

If you like to discuss a specific topic, please open an issue (if not yet done) and reference to this pull request.
I'd be very happy if you decide to help me with this release. Please send pull requests to the `release-1.0` branch. If merged, they will appear here. Thanks in advance. :)