Introduce send_multi and send_prepared

[majek]

As discussed here:

https://groups.google.com/d/msg/sockjs/vsFvHqppq5g/XQPgErs9BpUJ

We should introduce three new calls:

sockjs:send_multi([Conn], iolist)
sockjs:prepare(iolist)
Conn:send_prepared(prepared)

Of course send_multi might use prepared underneath.

[gleber]

I'll prepare a pull request when I have some time (I need to separate some of our own code from sockjs code for that and I'm very short on time). If someone needs it faster, please let me know.

There's still unsolved issue with fmt_* functions - @majek, you will probably know a nicer solution to that problem than me :)

[denen99]

Is there a status on this? Would be curious to help where I can, just not sure what has been done.