Added plist for easier startup of solr

Based on the guide at http://realityloop.com/blog/2011/07/19/setting-multicore-apache-solr-os-x-using-homebrew I modified the formula to include a plist for easier startup and usage of solr.

It is probably arguable what kind of configuration should go into the plist. I have tried to settle on what would be generally usable based on experience from my own uses cases (thats also why this is a pull request on [solr36](https://github.com/Homebrew/homebrew-versions/blob/master/solr36.rb) - similar addition could be made to [solr14](https://github.com/Homebrew/homebrew-versions/blob/master/solr14.rb) and [solr](https://github.com/Homebrew/homebrew/blob/master/Library/Formula/solr.rb)). But feel free to give your feedback and thoughts.

I have added a multicore configuration and installing it to `/usr/local/etc/solr36/multicore` because it will let users add several cores and keep them when the formula is upgraded.

As an example I added my index in `/usr/local/etc/solr36/multicore/my-project` and added `my-project` to `/usr/local/etc/solr36/multicore/solr.xml`. The configuration and index will be preserved if solr36 is uninstalled, reinstalled or upgraded.