use bindings to search for addons

For local development I'm using a debug build of node, which results in the following when trying to use this library.

```
npm test

> bunyan-syslog@0.2.2 test /Users/tmpvar/work/js/node-bunyan-syslog
> tap test


module.js:340
    throw err;
          ^
Error: Cannot find module '../build/Release/syslog'
...
```

and so on. This patch fixes the immediate issue of not being able to find the binding, the other test failures will be added in a separate issue.