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.
Travis-ci: 
License: MIT Licensed
UltimateCore uses gradle to build. Just clone UltimateCore and run the build.gradle like all other gradle projects.
You can use JitPack to build against UltimateCore using maven or gradle. Take a look at https://jitpack.io/#Bammerbom/UltimateCore