Add a new command to the CLI, get-gradle

This lists a subset of the known information about all gradle installations, formatted as JSON.  On the jenkins server, this information is stored in the file hudson.plugins.gradle.Gradle.xml.  What we are outputting here is a map, where the keys are the names of all of the gradle installations, and the values are an array of id's for all of the installers that correspond to the given installation.