Add video file entity

Run:
- php app/console doctrine:database:create
- php app/console doctrine:schema:create

Custom methods:
- isEncoded()
- hasError()
Add doctrine fixtures bundle

Run php composer.phar install.
Add video file data fixtures
Change upload folder name

The folder upload/videos will actually contain the encoded videos. Video uploaded through xhtml will be dumped to the xhtml folder.
Add video file fixtures

Add two video files in the fixtures directory. These files are used
when loading the data fixtures into the database: they are copied into
the public web directory and used to insert fake rows in the database.
Update .gitignore

Prevent the videos in the upload folder to be pushed.