quick fix for #152: allow users to style dialog types individually

I added the type to the class-name of alertify-dialog-inner. this way, the user can style only one dialog differently: 

    .alertify-dialog-inner-alert {
      background-color: red;
      padding: 25px; 
    }
    .alertify-dialog-inner-confirm,
    .alertify-dialog-inner-prompt {
      padding: 25px; 
    }

By the way: The grunt-build wasn't working for me: 

       Tracing dependencies for: alertify
       [Error: TypeError: Cannot read property '0' of null
           at Object.grunt.initConfig.requirejs.compile.options.onBuildWrite (<mypath>alertify.js/Gruntfile.js:115:44)
       ]

After adding a null-check, it worked fine enough to get build-results but the created `alertify.(min).js` had some errors.