Add css lint gem

@jongilbraith
Break the assets pipeline if css lin has errors
Add css lint options
Warnings doesn't break the assets pipeline
Move css lint options to a json file in the root

This follows the same approach as #283
Change options to settings

Name options is clashing
Fix typo
CssLint raise a CssLintError
Add csslint rake task
Avoid need to stub CssLint#initialize.
Add csslint_ruby gem
Ignore code between @codingStandardsIgnore annotations

This is related with #285

There are an open discussion about adding something like this to the csslint project github.com/CSSLint/csslint/issues/248
Move csslint duplicate-background-images from error to warning
Fix css errors
Ignore style guide ids
Remove sass commented lines
Comment css ignore code in csslint rake task
Merge branch 'css-lint'