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?
It's not defined before that, it just comes from the options, so that's a good point. It's another bug, so I'll add the union change to this PR.