Support running executables from Bundler gems and project-local scripts

This feature needs to be cleaned up and refactored, but that would require refactoring some other pieces. If this is merged, I'm going to do that in a separate pull request so we can have a more critical look at the effect of the refactoring, since none of the Java portions of this gem have any automated tests.

This change enables the following:

```
$ java -jar app.war -S script/rails console development
Loading development environment (Rails 3.2.12)
irb(main):001:0>
```

It builds on the work from #130 and #136.