Update the README.md and the ngSpinkitImagePreloader module

1. I've updated the paragraph 1 in the README.md. I believe it will be helpful for users to install component and update the bower.json using the only one command. Also popular projects do this in the same way (angular-ui/ui-router, angular-ui/bootstrap, grevory/angular-local-storage and etc).
2. I've also updated the spinkitImagePreloader a little bit. Just added the spinkit-image-preloader-class attribute in order to have ability configure the spinner-wrapper css-class. I also mentioned this feature in the readme.