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 if id's for all of the
installers that correspond to the given installation.
The original source for this info is
GradleInstallation.DescriptorImpl.getInstallations().
This commit includes a bunch of commented out stuff as well that could serve
as a future reference for navigating through more of the data structure, if it
was desired for the output to be more comprehensive.  The next commit will
clean this up.
Cleanup only.  No functional changes.
Add a descriptive comment, remove commented out code.
add an optional option, --name, to limit the output to just a single installation.
also add more and better comments and a little other misc. cleanup.
and add a --help option for a usage statement, but that still needs a little more work.
call printUsage() instead of printUsageSummary().
this was my plan all along, i just needed to figure out where to get a CmdLineParser object to pass.
i think what i'm doing to create it is okay, and it seems to work, but it was somewhat of a guess.

one slight difference between this case (calling --help explicitly) and the
implicit usage (e.g. when you get the command wrong) is that in this case
we're only printing out the options for this command, but in the other case
the general options to the jenkins CLI are also output.  i can't decide
whether i'd want those or not, but i don't know how to control them anyway, so
it's fine how it is now.
Merge pull request #19 from richfromm/master

 Add a new command to the CLI, get-gradle