Add some more improvements

[thiamsantos]

CHANGELOG

• Expose the proper file on main of package.json.
• Remove unnecessary jshint config since that standard is the code style.
• Remove version numbers of unpkg links on readme.
• Add npm script that minify and add the vendor prefixes (if the browser that doesn't support the property is used for more then 1% of global population. Using cssnano as build tool.
• Remove vendor prefixes of css source since that the script npm run build:css adds the necessary vendor prefixes.
• Add npm script that minify the js code. Using UglifyJS as build tool.
• Remove Grunt file and dependecies, because the npm script handle the build cases and decrease its complexity.

[thinker3197]

LGTM!