Add subscribers autowiring feature

[unkind]

This is a draft of autowiring feature described in #39.

Some notes:

• for consistency with regular subscribes_to, this feature doesn't explicitly disallows non-final events; one might think that if you type-hinted against an interface or abstract/inheritable event class, you'd also subscribe all the children, but that's not the case;
• since event_name_resolver_strategy is a global flag, I couldn't find way to test feature in the SmokeTest, so it has separate test case.

Example of configuration:

services:
    send_email_verification_instructions_subscriber:
        class: Acme\SendEmailVerificationInstructionsSubscriber
        tags:
            - { name: event_subscriber_autowired }

[coveralls]

Coverage decreased (-6.1%) to 83.382% when pulling 3391848 on unkind:feature-autowiring into 7879503 on SimpleBus:master.

[cmodijk]

I'm going close this in favor of #43 @unkind Tnx for your work and suggestions.