[Security] Add an option to disable the hasPreviousSession() check in AbstractAuthenticationListener
make scheme and method requirements first-class citizen in Route
added scheme and method setter in RouteCollection
made schemes and methods available in YamlFileLoader
made schemes and methods available in XmlFileLoader

it uses an attribute list instead of multiple scheme/method elements that I also experimented with
[Routing] add support for path-relative and scheme-relative URL generation
[Routing] added tests for generating relative paths and network paths
[Routing] made reference type fully BC and improved phpdoc considerably
fix phpdoc of UrlGeneratorInterface that missed some exceptions and improve language of exception message
[Form] Prevent trigger of E_USER_DEPRECATED for new API
[HttpFoundation] Update docblock for non-working method
[Form] Fix for `DateTimeToStringTransformer`
updated CHANGELOG for 2.0.21
updated VERSION for 2.0.21
updated CHANGELOG for 2.1.6
updated VERSION for 2.1.6
bumped Symfony version to 2.1.7-DEV
bumped Symfony version to 2.0.21-DEV
add preferred_query option to ModelType

This enables the ModelChoiceList to use 'preferred_choices' of the parent ChoiceType.
use preferred_choices in favor of preferred_query