Add/volatile support

[kevin-roark]

this seems like a solid feature to me. Let me know what y'all think!

[kevin-roark]

also, I was only able to run zuul tests locally because my internet connection is really wonky right now

[kevin-roark]

addresses a very old issue here #283

[rauchg]

The underlying socket should be still writable, but if busy writing out packets we need to ditch the packet.

[rauchg]

This is why I added a callback to the engine.io writes btw. The idea was to match the write signature from Node.JS, that takes a callback when the packet is being written out: http://nodejs.org/api/net.html#net_socket_write_data_encoding_callback

This means that we can still swap the underlying transport and support packet volatility.

[kevin-roark]

@guille are you saying ditch the check for engine.writable, and have a separate property that is controlled in the write callback?

[rauchg]

[kevin-roark]

@guille let me know how you feel now 🍪. also, the should not autoconnect test is failing for me, also in upstream. uh-oh!!

[rauchg]

I'm a little concerned about the testing strategy relying on the knowledge of a private member (numPacketsWriting), and altering it from the outside. Maybe a test that doesn't rely on this?