Added GUI for managing and discovering Xcode Plugins

Long time I have wanted to develop a plugin repository and management for Xcode.
I'm glad i found JDPluginManager - this is what I was looking for in managing Xcode plugins. 

After speaking to Xcode developers I've discovered they are not aware of the growing number of helpful Xcode Plugins that open sourced and on Github.

So I used JDPluginManager for managing the installed plugins and wrapped it up to a nice GUI interface that also includes available plugins:
![screenshot6](https://f.cloud.github.com/assets/4062328/376963/8c6b8104-a468-11e2-99e9-2f9fac37a82d.png)
The plugins are maintained in the `plugins.json` and everyone can add a new plugin there by forking and pull request.

The plugin also fetches more data about each of the plugins from Github.
This fetching is happens upon request when the plugin manager screen is requested, so it wont slow the loading of Xcode. 
For each plugin it:
- Fetches and displays description from Ggithub
- Fetches and displays last push from Github so it can know if an update is available by comparing when was the plugin modified locally.
![Screen Shot 2013-04-13 at 9 46 02 PM](https://f.cloud.github.com/assets/4062328/376977/74ed87a0-a46a-11e2-9278-10a7dd12e0b4.png)

More Ideas: 
- Hosting the plugins.json so it can fetched if changed - today it can be done using the regular plugin update.
- Displaying the README.md in Webview window by converting the md to html

--Danny