New csslint task and some code refactor

A new csslint task has been added. It uses `grunt-contrib-csslint`
to lint css files in the dist directory.Loading it and defining
its tasks has been done in "root/Gruntfile.js".

The tasks `csslint` and `jshint` require to have configuration
files. "root/.csslintrc" and "root/.jshintrc" have been added
for this.

A new task `test` has been added that could be used to run both
jshint and csslint.

Minor errors in installation instructions in "README.md" have been
corrected.