Update HorseStats for 1.7.10

I made a few changes to allow the plugin to work with 1.7.10 as well as adding an updated gitignore file, changing version to 3.1.1 and changing the Maven repo to sk89q's. Of course, this will mean dropping 1.7.9 support for now, so pull as you wish - but as I said I could also help out with an implementation that would allow the plugin to run cross-version.