drop dependency on doctrine/reflection

- add ErrorHandler component
- drop dependency on fig/link-util
- raise dependency on symfony/contracts 1.1.8
- raise dependency on doctrine/doctrine-bundle 1.5
- raise dependency on monolog/monolog 1.25.1
- allow twig 3 (review #1774967)
- allow doctrine-bundle 2 (review #1775102)
- add dependency on sodium extension
- add dependency on guzzlehttp/promises

add HttpClient, Mailer and Mime components
use range dependencies
raise dependency on symfony/contracts 1.1.3
raise dependency on symfony/polyfill 1.11
raise dependency on doctrine/annotations 1.7
raise dependency on egulias/email-validator 2.1.10
add dependency on masterminds/html5
add dependency on mongodb/mongodb
add dependency on nyholm/psr7
add dependency on psr/http-client