Tweaks to gitignore

* adds `_site` and `_posts` to avoid shenanigans when switching from
  `gh-pages` branch
Add ES6 Import Validation

- Add dependency broccoli-es6-import-validate
- Add validateJs tree (only when env !== 'production')
Merge pull request #285 from twokul/gitignore

Tweaks to gitignore
[fixes #302] although location auto requires server side support, I would like to push for it as the default and aim to provide the appropriate server side components to make it seamless.
document.location.origin is not needed, and fails in IE
Merge pull request #209 from jgable/es6ImportValidate

Add ES6 Import Validation
Improve CSS preprocessors situation
Merge pull request #317 from HeroicEric/fix-css-preprocessors

Improve CSS preprocessors situation
Update README.md
Restructure internals: Introduce fixed structure commands and tasks
80 chars
Cleaned up findProject()
Make loadCommands() a bit nicer
Assert that files export a task/command
Align string
Align bower
alignment
remove old-cli
use shortened URL
help and init can be run anywhere