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?
> You showed in one of the screenshots that the issue is with only using the __dirname without the path.relative(process.cwd().... Then you posted the screenshot with it working correctly.

Correct, it doesn't work with `path.relative(process.cwd()`. 

> union works fine because it flattens the arrays internally. 

`_.union` only uniquifies, it doesn't flatten.

> there was some weird timing with what I was referring to there.

k