Added '@@' escaping strategy for YamlFileLoader and YamlDumper

Added the possibility to to use '@@' as an escaping strategy for
parameters that should be treated as strings but start with '@'
(i.e. safe mailer passwords).
Merge branch '2.1' into 2.2

* 2.1:
  #7106 - fix for ZTS builds
  Added '@@' escaping strategy for YamlFileLoader and YamlDumper
  [Yaml] fixed bugs with folded scalar parsing
  [Form] made DefaultCsrfProvider using session_status() when available
  Added unit tests to Dumper
  Update .travis.yml (closes #7355)
  [HttpFoudantion] fixed Request::getPreferredLanguage()
  Revert "merged branch jfsimon/issue-6928 (PR #7378)"
  Routing issue with installation in a sub-directory ref: https://github.com/symfony/symfony/issues/7129

Conflicts:
	.travis.yml
	src/Symfony/Bundle/FrameworkBundle/Routing/Router.php
	src/Symfony/Component/Routing/RouteCollection.php
Merge branch '2.2'

* 2.2:
  #7106 - fix for ZTS builds
  Added '@@' escaping strategy for YamlFileLoader and YamlDumper
  [Yaml] fixed bugs with folded scalar parsing
  [Form] made DefaultCsrfProvider using session_status() when available
  Added unit tests to Dumper
  Update .travis.yml (closes #7355)
  [HttpFoudantion] fixed Request::getPreferredLanguage()
  Revert "merged branch jfsimon/issue-6928 (PR #7378)"
  Routing issue with installation in a sub-directory ref: https://github.com/symfony/symfony/issues/7129