feat(logs): introduce logging for symfony

[Litarnus]

Introduces Sentry logging for Symfony.

Configuration works similar to the BreadcrumbHandler.

It's necessary to define the LogsHandler as a service like this:

services:
  Sentry\SentryBundle\Monolog\LogsHandler:
    arguments:
      - !php/const Monolog\Logger::INFO

and the register the handler in your monolog handler config:

monolog:
  handlers:
    sentry_logs:
      type: service
      id: Sentry\SentryBundle\Monolog\LogsHandler

and finally the option itself needs to be enabled:

sentry:
  options:
    enable_logs: true

This will end up in the docs very soon, but I wanted to provide this information here as well in case there are any concerns.

closes #925
closes SYMFONY-17

[Litarnus]

I decided to go with inheritance here vs composition because I didn't want to write tests for all methods that are just forwarding.

[linear]

SYMFONY-17 Sentry Structured Logging for Symfony