Base loader for pure JS app

* defines a bas class and hooks an  ajax action in press-this.php
* press-this.html: base HTML file/loader
* load.js: loaded by press-this.html after hosted jQuery
* config-*.js: contextual app config files loaded by load.js based on
if bookmarklet or extension
* app.js: loaded by load.js once configs are primed, app logic goes
here.

You can test the base work by activating the Press This plugin in WP,
then going directly to
http://your_site/wp-content/plugins/press-this/press-this.html