Refactor jshint files.

We'll use grunt, and jshintignore is useless.
Remove comments in jshintrc to be parsed in grunt.
Add node_modules in gitignore.
Remove build shell script.

We'll use grunt to build the scripts.
The shell script is no more used.
Csslint the css file.
Add csslintrc.
Add files for Grunt.
Minify source files with the new build system.

From now, the build can be done with 'grunt build'.
Add CONTRIBUTING document.