Use debug add-on if release add-on is unavailable.

Try to load the release build first before trying the debug build.
Fixes add-on loading for people that have npm_config_debug set in
their environment or use a debug build of node.

Fixes #36.
Add `npm test` target

Nothing special, it just executes `node test.js`.
Code cleanup, use named imports
Replace tabs with two spaces
Make compatible with v0.11

Use a smidgen of macro magic to abstract away the differences between
node.js v0.10 and v0.11.  In particular, abstract the differences
between V8 3.14 and 3.24.

Note that this commit targets the current HEAD of node.js master, it's
not compatible with the most recent v0.11 release, v0.11.12.

Fixes #56.