Implements #207, Last-Modified headers in response via annotation
fixed and simplified previous merge about the LastModified tag
changed the @LastModified tag to use expressions
merged @LastModified tag into the main @Cache one
added Etag support to the @Cache annotation
added the @Security annotation
updated CHANGELOG and bumped versions of deps
added some suggestion in composer
removed dev stability
feature #238 First steps towards 3.0 ... or expressions everywhere ;) (fabpot)

This PR was merged into the master branch.

Discussion
----------

First steps towards 3.0 ... or expressions everywhere ;)

The 3.0 version of this bundle will be the first one to not be aligned with the released of Symfony itself.

That said, the major changes provided by this PR are support for the new `@Security` annotation and the HTTP validation headers support in the `@Cache` annotation:

```php
/**
 * @Route("/post/{id}")
 * @Cache(smaxage="15", lastModified="post.getUpdatedAt()", ETag="'Post' ~ post.getId() ~ post.getUpdatedAt()")
 * @Security("has_role('ROLE_ADMIN')")
 */
public function showAction(Post $post)
{
}
```

As you can see, the `lastModified` and `Etag` attributes of the `@Cache` annotation allows the developer to define an expression that must be used to set the `Last-Modified` and the `ETag` headers. In addition, it automatically takes care of the 304 management (which frees you from copy/pasting the same boiler-plate code over and over).

The `@Security` annotation takes an expression like the ones supported by the access control rules. It performs better that the access control rules as it is only executed when the route matches.

You can find more information in the updated documentation.

Commits
-------

12813c0 added some suggestion in composer
7455490 updated CHANGELOG and bumped versions of deps
82182d1 added the @Security annotation
af45961 added Etag support to the @Cache annotation
ffacde8 merged @LastModified tag into the main @Cache one
31310d1 changed the @LastModified tag to use expressions
68525ca fixed and simplified previous merge about the LastModified tag
8dbea9c Implements #207, Last-Modified headers in response via annotation
fixed composer.json