Restore compile and run functionality

@ctrueden and I have been discussing this in the context of the current [deployment goals](http://imagej.net/2015-06-15_-_Major_updates_in_the_works).

With the changes to [file structure](https://github.com/imagej/imagej/issues/101) and second look at making [install work](imagej/scijava-issue-shuttle#3) in the script editor, I think it would be nice to get a minimal version of `Compile and Run` working again.

My proposal is to use a dedicated class loader that is re-created during each execution, and only support one-off plugins.

Start by reviewing the script editor's current file structure and the linked deployment and installation discussions. The intended result is a minimal Compile and Run flow using a class loader recreated for each execution and supporting only one-off plugins.

Written by the indexing model from the issue text.