define :gen_event functions

[mitchellhenke]

Related to #237

The Elixir GenEvent Behaviour had default implementations of these functions: https://github.com/elixir-lang/elixir/blob/v1.5.2/lib/elixir/lib/gen_event.ex#L117-L129

and erlang gives some warnings now because of that:

warning: undefined behaviour function code_change/3 (for behaviour :gen_event)
  lib/sentry/logger.ex:1

warning: undefined behaviour function handle_info/2 (for behaviour :gen_event)
  lib/sentry/logger.ex:1

warning: undefined behaviour function terminate/2 (for behaviour :gen_event)
  lib/sentry/logger.ex:1

So I defined them