Use util.inherits instead of custom inherits function

Hey, there's one of those in node core! 

Unfortunately, I wasn't able to run the tests on my machine since it failed to compile... 

```
> node-fit@0.0.0 install /Users/adunkman/projects/node-fit
> node-gyp rebuild

Traceback (most recent call last):
  File "/Users/adunkman/.node-gyp/0.8.14/tools/gyp/gyp", line 18, in <module>
    sys.exit(gyp.main(sys.argv[1:]))
  File "/Users/adunkman/.node-gyp/0.8.14/tools/gyp/pylib/gyp/__init__.py", line 511, in main
    return gyp_main(args)
  File "/Users/adunkman/.node-gyp/0.8.14/tools/gyp/pylib/gyp/__init__.py", line 494, in gyp_main
    options.circular_check)
  File "/Users/adunkman/.node-gyp/0.8.14/tools/gyp/pylib/gyp/__init__.py", line 133, in Load
    depth, generator_input_info, check, circular_check)
  File "/Users/adunkman/.node-gyp/0.8.14/tools/gyp/pylib/gyp/input.py", line 2378, in Load
    depth, check)
  File "/Users/adunkman/.node-gyp/0.8.14/tools/gyp/pylib/gyp/input.py", line 358, in LoadTargetBuildFile
    includes, True, check)
  File "/Users/adunkman/.node-gyp/0.8.14/tools/gyp/pylib/gyp/input.py", line 216, in LoadOneBuildFile
    None)
  File "binding.gyp", line 1
    {
     ^
SyntaxError: invalid syntax
gyp ERR! configure error 
gyp ERR! stack Error: `gyp` failed with exit code: 1
gyp ERR! stack     at ChildProcess.onCpExit (/usr/local/lib/node_modules/npm/node_modules/node-gyp/lib/configure.js:350:16)
gyp ERR! stack     at ChildProcess.EventEmitter.emit (events.js:99:17)
gyp ERR! stack     at Process._handle.onexit (child_process.js:678:10)
gyp ERR! System Darwin 10.8.0
gyp ERR! command "node" "/usr/local/lib/node_modules/npm/node_modules/node-gyp/bin/node-gyp.js" "rebuild"
gyp ERR! cwd /Users/adunkman/projects/node-fit
gyp ERR! node -v v0.8.14
gyp ERR! node-gyp -v v0.7.1
gyp ERR! not ok 
npm ERR! node-fit@0.0.0 install: `node-gyp rebuild`
npm ERR! `sh "-c" "node-gyp rebuild"` failed with 1
npm ERR! 
npm ERR! Failed at the node-fit@0.0.0 install script.
npm ERR! This is most likely a problem with the node-fit package,
npm ERR! not with npm itself.
npm ERR! Tell the author that this fails on your system:
npm ERR!     node-gyp rebuild
npm ERR! You can get their info via:
npm ERR!     npm owner ls node-fit
npm ERR! There is likely additional logging output above.

npm ERR! System Darwin 10.8.0
npm ERR! command "node" "/usr/local/bin/npm" "install"
npm ERR! cwd /Users/adunkman/projects/node-fit
npm ERR! node -v v0.8.14
npm ERR! npm -v 1.1.65
npm ERR! code ELIFECYCLE
npm ERR! 
npm ERR! Additional logging details can be found in:
npm ERR!     /Users/adunkman/projects/node-fit/npm-debug.log
npm ERR! not ok code 0
```

Travis, it'd be awesome if you told me if I broke anything!