api: introduce net.Listener and net.Conn wrappers

[pires]

This is an attempt to partially address #3 with pieces of #2. The reason why I'm not merging #2 (or the code I have here) is because I don't feel confident enough it isn't introducing breaking changes to any existing users of this library who may be pulling from master branch.

@Freeaqingme @mgouline @elico @anisse can you PTAL?

TODO

• fix golint output
• add test(s) for any added functions wherever makes sense
• add net.Dial example in order to fix An example for a Dialer and Listener is required. #3

[coveralls]

Coverage decreased (-2.9%) to 86.902% when pulling 265cd46 on pires/pr_2_rework into 9c0bafa on master.

[anisse]

LGTM, especially with the updated comments from the later commits.

It seems this implementation is better than my own wrapper, which is a bit more naive (https://github.com/anisse/proxylistener/blob/master/proxylistener.go) :

• initially I really liked the idea of wrapping net.Listen() directly as I did, but I realize now it's much better to wrap a listener, which is an interface so it can be something else than what stdlib provides. I also allows passing a tuned tcplistener (with setdeadline for example)
• In addition, it gives you better control with the additional read timeout, which allows having different behaviour than just the accept timeout (which my implementation does not allow anyway)
• I also didn't really like the use of sync.Once, but I see now that it makes sense in the context of having a Conn that can be used in the client and server case (mine only does the server case).

In conclusion, LGTM 👍