Add support for scoped npm modules

[doctyper]

Hello,

• This pull request introduces support for scoped npm modules. We currently have a need for this in my organization. Nutshell: Slush is now able to find modules like @company/slush-foo.
• I have also added the ability to detect paths via process.env.NODE_PATH in an effort to avoid hard-coded paths.
• I've also stopped converting paths to lower case, to address Issues with OSX and case sensitive filesystem #17.
• Lastly, I've fixed a failing test (it was failing before my edits).

I'd love to see this land soon! Let me know if you have issues / questions / concerns.

[joakimbeng]

@doctyper great work! Thanks!