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?
also, returning paths that are relative to the cwd is confusing to the user sometimes. especially if the path being resolved is actually in the cwd, or one folder deep.