[Security] Add SimpleForm authentication
[Security] Add simple_token auth method
[Security] Renamed simple_token to simple_http, added support for failure and success handler to both simple firewalls
fixed wrong Logger interface
moved the simple HTTP authenticator to a pre-auth one
[Security] allowed simple pre-auth to be optional if another auth mechanism already authenticated the user
marked some classes as being experimental in 2.3
[HttpFoundation][NamespacedAttributeBag] Refactoring of resolveKey() method
[Console] Added status code to CommandTester and ApplicationTester

Theses classes are already statefull. They contain the input and the
output data. So we can safely add the last status code to the class.
Fix Crawler::children() to not trigger a notice for childless node
merged branch lazyhammer/issue-8048 (PR #8054)

This PR was merged into the 2.1 branch.

Discussion
----------

[DomCrawler] Fix Crawler::children() to not trigger a notice for childless node

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #8048
| License       | MIT
| Doc PR        | n/a

Commits
-------

91b8490 Fix Crawler::children() to not trigger a notice for childless node
[TwigBundle] Added missing files for phpunit
updated CHANGELOG for 2.3.0-RC1
updated VERSION for 2.3.0-RC1
bumped Symfony version to 2.3.0-DEV-DEV
[CssSelector] Fix :nth-last-child() translation
[CssSelector] Fix :nth-last-child() translation
[Console] Add clear() to ProgressHelper.

One may want to print something else while the progress bar is running.
The output will be messy if the progress bar is not removed first.

One may also want to remove the progress bar after the work is complete.
[Console] fix and refactor exit code handling
[Intl] Fixed the class namespaces in the @covers annotations.