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?
> The "ugly" directory is coming from inside resolve-dep.. 

Hmm, by "ugly" do you mean "correct"? e.g. fully expanded, absolute, non-shortened filepaths?