Make loadPath relative to gulpfile cwd

This is in response to #75 (and similar).

It makes all paths passed to loadPath relative to procDir, which brings the behaviour of gulp-ruby-sass more in line with sass itself.

In the latest version, you either have to add all the sass files that your sass files @import to the .src glob (meaning you have to specify them twice - once in the glob, once in the sass) or you have to use absolute paths in loadPath, which is not really viable in codebases that are cloned across multiple machines.

The expected behaviour in the sass tool itself is that load_paths [defaults to the working directory](http://sass-lang.com/documentation/file.SASS_REFERENCE.html#load_paths-option).

I realise this changes the behaviour of the option, not sure if that's too much. I'd find this very useful though, so I thought I'd suggest it and see what you thought.
Those are all good points :smile:. Sass is naturally on disk so it takes a little cleverness and a little configuration to make it work as part of the stream. But that's what the plugin is for, and as such I think it needs to have a focus on working with the stream. If a user wants to work outside of the streaming build sys they can always spawn a `sass` command directly in their gulpfile.

In your above example you can use a glob like `gulp.src(["styles/**/*scss", "bower_components/**/*.scss"], { base: '.'}). Pretty easy. The base path thing could be confusing, but I'd rather document and have people learn how gulp and globbing works than magic it away for them.

My other issue with loadpath reliance is it breaks sourcemaps, which are barely hanging together. I'd rather discourage loadpaths as a default for that reason as well.

What do you think?