Extract a "filer" interface for listeners

[russellhaering]

Manners currently has a type switch that it uses to get a file descriptor from various types of listeners:

1. Listeners from the stdlib net package which have a File() (*os.File, error) method.
2. Internally defined listener types which mostly wrap net listener types.

This pull request defines a filer interface which is implemented by all currently supported net listeners. It also two internally defined listener types to implement this interface.

Aside from being a cute way to use Go's type system, this makes it possible for external packages to pass in custom net.Listener implementations as long as they have an appropriate File() method. As before, listeners which do not will experience failures when calling GetFile() on a GracefulListener.

Two possibilities I chose not to pursue because of their broader implications for the Manners interface:

1. Make GracefulListener implement the filer interface itself by aliasing GetFile() as File().
2. Make GracefulListener only accept net.Listeners which also implement filer (probably by making the filer interface an explicit superset of net.Listener).

[pquerna]

LGTM