Maven's fine with it, but it is bad practice.
OpenInv still modifies plugin.yml in OpenInvPlugin. It should really be done via the Assembly Plugin, but it works and I am lazy.
Mostly based on Mbaxter's AbstractionExamplePlugin
Compiling OpenInv for a specific version is very easy - just compile the correct module.
Compiling for a set of versions is slightly more complex. You'll need to use a profile for the versions you want to compile. Provided profiles are latest, modern - versions 1.7.10+, and all. For more information, check out the Maven guide http://maven.apache.org/guides/introduction/introduction-to-profiles.html
This commit doesn't change anything with the plugin itself, but it makes it loads easier for people to maintain and compile their own version without destroying backwards compatibility.
