allow event filters

[sumitbirla]

This change allows one to install event filters for inbound listener:

Librevox.start SomeListener, :filters => ["Event-Name CHANNEL_EXECUTE", "Event-NAME HEARTBEAT"]

[vangberg]

Thanks for your work!

Wouldn't a specific listener always want the same filter? That is, would it not make more sense to specify them as part of the listener, instead of Librevox.start/run? Something like:

class MyListener < Librevox::Listener::Inbound
  filters 'Header' => 'Value', 'Foo' => 'bar'
end

Thoughts?