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?
When I pass in this path to resolve-dep: `/Users/doowb/work/assemble/support/assemble-v0.4.39/node_modules/assemble/lib/plugins/page-collection-properties.js` and I get an error that says: `Error resolving /Users/doowb/work/assemble/support/assemble-v0.4.39/Users/doowb/work/assemble/support/assemble-v0.4.39/node_modules/assemble/lib/plugins/page-collection-properties.js`

I don't think that's correct because the `process.cwd()` is being prepended to the absolute path