UltimateCore is an essentials plugin which aims to be simple, powerful and configurable. It has an extensive API, all messages are configurable and is easy to use.
UltimateCore uses gradle to build. Just clone UltimateCore and run the build.gradle like all other gradle projects.
Using UltimateCore as Library
You can use JitPack to build against UltimateCore using maven or gradle. Take a look at https://jitpack.io/#Bammerbom/UltimateCore