Load Bootstrap via Bower

TODO:
- Update README
- Use grunt-bower-task?
Merge branch 'bower' into 7.0.0
Specific Bootstrap version
Load jQuery from Bower
Load Modernizr from Bower, lean build with Grunt
Better LESS organization
Ref #867 - Add Respond.js (not enqueued by default)
Use grunt-wp-assets for version task

* default use filename version revving instead of querystring
* versioning file will be ignore
Merge pull request #990 from hariadi/7.0.0-grunt-wp-asset

Use grunt-wp-assets for version task
Run bower install after npm install
Fix parseFiles in grunt-modernizr task
Update clean task for versioned assets
Use latest Bootstrap
Ref #933 - dev and build tasks for Grunt

Dev: don't minify
Build: minify, lean Modernizr build

Dev assets will load if you have defined your WP_ENV as development
If you don't use Bedrock, you'll need to add this to your wp-config.php:

define('WP_ENV', 'development');

TODO: lib/scripts.php shouldn't need to be committed for changes after
running the version task
Use load-grunt-tasks
Replace grunt-wp-assets with grunt-filerev and grunt-filerev-assets

By writing to assets-manifest.json with grunt-filerev-assets, it's
no longer necessary to commit changes to lib/scripts.php when developing
Clean up the latest changes to lib/scripts.php

:blue_heart: @swalkinshaw
Add .gitkeep to assets/js/plugins
Close #992 - Move some Roots features into Soil
Update screenshot resolution to 880x660, change to blank image