src: convert to NAN for Node 0.8->0.11+ compat
doc: add README

Update Socket#send() in src/unix_dgram.js so it complies with the
documentation; there is no reason to make the callback mandatory
so make it optional.
src: move unix_dgram.js to lib/
src, lib: get rid of js errno global

Return error codes directly rather than assigning them to the errno
JS global.

I made node.js core stop doing that in joyent/node@12d0f0b and then
some more in joyent/node@ca9eb71, make unix-dgram follow suit.
src: rename sun -> s

@aklt reported compilation errors on Solaris because of a conflicting
definition of 'sun'.  I've never seen that myself but it doesn't sound
implausible and it's trivial to change.

Hopefully fixes #7.
src: rename sun -> s redux

Like commit 05fccae but for another call site.  Overlooked in the
previous change, mea culpa.
src: fix use after free

The memory of the uv_poll_t handle was freed without the handle being
closed first.  As a result, it was still referenced internally by libuv,
leading to spurious segmentation faults.  This looks like an oversight
in the switch from libev to libuv in commit 04dd6d7.

You'd think _I_ of all people would know better, wouldn't you?

Fixes #8.
src: upgrade to nan 0.8.0

Should make it possible to compile against node.js v0.11.12.

Fixes #11.