Core plugin path fix

uses the process.cwd to get the relative path to the core plugins so resolve-dep can find the correct directory from inside other projects. fixes #532

@jonschlinkert (or anyone else on Windows) will you pull this down and test it in a project using assemble to make sure the paths are correct for Windows?
however, this:

![image](https://cloud.githubusercontent.com/assets/383994/3374165/0bd9f998-fbbb-11e3-95ce-efda9e7f74de.png)

returns this:

![image](https://cloud.githubusercontent.com/assets/383994/3374171/14fbbfac-fbbb-11e3-8170-149ca695a8ea.png)