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?
to emphasize, the op said this:

> The ugly thing is that my directory, where my grunfile is located, is listed twice in the error message.

e.g. a string is being concatenated.