Better integration with CircleCI

[lookfirst]

CircleCI cache's the .m2 folder before it runs a deployment. This means that if I run mvn appengine:update, this plugin will download the entire AppEngine SDK zip file every time my build runs. That is a relatively slow 16mb file download that bloats my CI build times.

CircleCI tries to be smart and run mvn dependency:go-offline before it cache's the .m2 folder. If somehow the resolveAndSetSdkRoot() method could be run during the dependency:go-offline phase, that would be awesome. If not, I ask that you create a mvn appengine:go-offline mode which just runs that method.