Add scoped helpers environments from file-utils

Base generator now expose `.src` and `.dest` who're file-utils
environment scope as the sourceRoot and destinationRoot. This will allow
access to more file helpers method, and also more transparent process
for filtering, transpiling and validation (conflicter).

This is a follow up commit after #313 discussion. As Grunt team ain't yet moving on exporting Grunt.file into a standalone module, it felt important to me we moved on this one as it felt a major piece for future generator user customization (style guide, JS || coffeescript, etc).

The standalone module is here: https://github.com/SBoudrias/file-utils  
And I implemented a simple demo on Generator-bbb: https://github.com/backbone-boilerplate/generator-bbb/compare/remove-grunt (Generator-bbb was using grunt.file so the file methods stay the same and ain't shown clearly in the diff, but the filters are used extensively to write file in the correct style guide).