solr: add startup plist

Added a plist startup like was previously done in [solr36](https://github.com/Homebrew/homebrew-versions/commit/4fa8b9b3c8f37667f1d993a53a6b9da383d87179) at Homebrew/homebrew-versions#366.

Defaults to running Solr on port 8984 so it can run in parallel with solr36 which runs on port 8983.
Ah. Was it this change: https://github.com/Homebrew/homebrew/commit/6cf528545e74c4b9c5f9a93b0e5a8d711a95f612#diff-2fc301c74cc197a899296b9c6efd2439R1

And does it only work when the install is directly on `etc` and `(etc+name+"multicore").install` as I do?

Bacause I don't get the warning if I change the formula to:
```ruby
    etc.install libexec+"example/multicore" => 'solr'
```

That moves the folders a bit around but I can make that work also.