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?
well, I'm trying to show you that the code you did with `process.cwd()` doesn't work. the concat thing _is_ the bug. not another bug. I think flattening with either concat or union would probably be better