New Feature: HTML Plugins

This Pull Request is a WIP.

The idea ist to place HTML files into the Plugin Folder and render them as tiny WebViews. The HTML should be allowed to execute JavaScript, make AJAX Requests, load local files(?)... etc.

What do you think?

Example:
![bitbar webview](https://f.cloud.github.com/assets/26716/1979878/52fcfbe4-83b7-11e3-9296-1b5b523192cd.png)