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?
where is `this.options.plugins` plugins defined before this? b/c if `this.options.plugins` returns a string, then it's concatenating a string to an array. hence: 

![image](https://cloud.githubusercontent.com/assets/383994/3374141/bc6a39fe-fbba-11e3-97e5-5c033c6b77fc.png)