2.0.0-alpha2

DO NOT USE THIS RELEASE

One of the BC break in this PR could cause corrupted objects to be created when unserializing the session, and got reverted. Use alpha3 instead.

• [BC break] The deprecated entity classes have been removed.
• The minimum requirement for Symfony has been bumped to 2.3 (older versions are already EOLed).
• [BC break] UserInterface::isUser has been removed as it was used only by the old validation logic removed a long time ago.
• [BC break] The FOSUserBundle:Security:login.html.twig template now receives an AuthenticationException in the error
  variable rather than an error message.
• [BC break] The templating engine configuration has been removed, as well as the related code.
• [BC break] Changed the XML namespace to http://friendsofsymfony.github.io/schema/dic/user
• [BC break] Added UserInterface::getId.
• [BC break][Reverted in alpha3] Removed unused properties expired and credentialsExpired including corresponding methods. This may break code,
  makes use of this methods, extending classes, and/or existing installations because of missing mappings for required db fields.