form to upload a plugin jar in plugin tab

This is the initial commit for "enable plugin install/uninstall on the UI" feature. Work in Progress. Please feel free to give us feedback along the way. 
Got around to seeing this. I'm no expert in UI/UX. But, I was wondering whether "Upload Plugins" needs to be given such a prominent spot on the page. It's at the top, and that would make sense if it's the most used user action. I don't know if it would be. This is what I mean:

![uploadplugin_topofpage](https://cloud.githubusercontent.com/assets/172235/5474264/ee02093a-85dd-11e4-997d-3141c1c7d47a.png)

Maybe you can consider doing something like [this](https://www.dropbox.com/s/90nwmsrzunrl6cq/UploadPlugin_WithButton.mov?dl=0)? I've uploaded the [index.html.erb](https://www.dropbox.com/s/bgxwbhed6v33mkl/index.html.erb?dl=0) I used to make that happen.