MDLSITE-3612 - support csslint checking of CSS

Since it reports in checkstyle format we can use existing reporting mechanisms much like the other changes.

You need to npm install -g csslint.

You can test it with this:
```
#!/bin/bash
set -e

export gitcmd=/usr/local/bin/git
export phpcmd=/usr/local/bin/php
export jshintcmd=/usr/local/bin/jshint
export csslintcmd=/usr/local/bin/csslint
export gitdir=/Users/danp/www/im
export WORKSPACE=/Users/danp/Desktop/workspace
export remote=git://github.com/danpoltawski/moodle.git
export branch=csslint-test1
export integrateto=master

./remote_branch_checker.sh
```

And there are these three tests:
1.  `git://github.com/danpoltawski/moodle.git csslint-test1` - this should not do any checking because there are no CSS files changed (you should see message in output)
2.  `git://github.com/danpoltawski/moodle.git csslint-test2` - this should run the linter with the .csslintrc config settings provided and you should see 1 error and 1 warning (duplicate rule and bad definition)
3.   `git://github.com/danpoltawski/moodle.git csslint-test3` - this should default to only the error checking as no csshintrc file exists in the repo, so you should see a message about it using only error checking and you should only get the parse error reported (not the duplicate rule)