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.
I actually removed the log stuff. It wasn't needed (I think it is in the `solr36` formula on which this pull request is based).

But I moved some other stuff to `post_install`.