Same baseDir for styles/scripts and preprocessors

@JeffreyWay I was answering a question on Laracasts' forum and it was not clear at first that Elixir assumes styles and scripts should go inside the `public` directory. Suppose I'm developing a Angular app, I would want to combine/minify all my `js` files and not have them available in the `public` directory, same for styles. This pull request fixes that. I've tested this changes manually for scripts/styles and preprocessors, everything is working fine.

I also suggest to assume `**/*.js` for `mix.scripts()` and `**/*.css` for `mix.styles()`, let me know what you think and I can add that to this pull request.