added a brew formula for nexus 2.3.0_04

This is a formula for sonatypes open source nexus server. The formulat will download an archive file and then install the main (nexus) sh script and link to it.
Once installed nexus can be started on the command line with the following command:

nexus { console | start | stop | restart | status | dump }

A plist is also generated into the install directory. The plist can be loaded and started with the launchctl tool. The plist will only take effect after a restart.