src: fix debug build add-on loading

Before this commit, the loader in src/unix_dgram.js only looked for the
release build add-on.  This commit makes it:

 * Fall back to the debug build if no release build is found, and

 * Removes the node-waf workaround.  Support for node-waf was removed
   in commit e7e3f71 so there is no point in keeping it around.

Fixes #5 and #9.
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.
Fixed sporadic segmentation fault under rhel x86_64 platform
Synced against upstream repo
Merge branch 'master' of github.com:bfrancojr/node-unix-dgram

Conflicts:
	lib/unix_dgram.js
	src/unix_dgram.cc