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?
and this:

![image](https://cloud.githubusercontent.com/assets/383994/3374187/316e2292-fbbb-11e3-8977-d11d7ba50c1a.png)

returns:

![image](https://cloud.githubusercontent.com/assets/383994/3374191/38c72ebc-fbbb-11e3-84dc-d7d6b2012699.png)