[SecurityBundle] added acl:set command

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | no
| License       | MIT
| Doc PR        | n/a

This new command allows to set ACL directly from the command line. This useful to quickly set up an environment and for debugging / maintenance purpose.

This PR also includes a functional test system for the ACL component. As an example, it is used to test the `acl:set` command.
The provided entity class is not mandatory (tests will still be green without it) but can be useful to test other ACL related things. I can remove it if necessary.

The instantiation of the `MaskBuilder` object is done in a separate method to be easily overridable to use a custom one (e.g. the SonataAdmin one).
Hi @dunglas, I'm redacting a "New in Symfony 2.6" blog post about the new `acl:set` and I wonder if this help message is correct. If you are using the `--class-scope`, you are granting access to all the objects of the same class, so the trailing object id (`42` in this case) should be removed, right?

Original help message:

`<info>php %command.full_name% --class-scope --user=Symfony/Component/Security/Core/User/User:anne OWNER Acme/MyClass:42</info>`

Proposed help message:

`<info>php %command.full_name% --class-scope --user=Symfony/Component/Security/Core/User/User:anne OWNER Acme/MyClass</info>`