Added migrations

[lsv]

• Added "real" migrations
• Tests works with both sqlite and mysql (and yes the blog_test.sqlite file are deleted), I can not test with postgres at the moment

What this "gives" to the demo, which I think would be lovely to see.

• It gives real migration, using doctrine migrations, (and populating the database with data on the fly) meaning that doctrine/fixtures actually is not needed anymore (only for tests)
  • Due to its not possible to inject services into migrations, it also showcases how to make a public service
• It also showcases how to test database in functional tests (creating database, deleting database, insert fixture data)

As discussed in #778

[lsv]

@yceruto as in #778 doctrine:fixtures:load should not be available in production mode, due to best practices.