Enable use of trackingSubmodules

trackingSubmodules were introduced in v1.8.2 of GIT as a way to always
track the tip of the branch you are interested in for a submodule.  This
is turned on via the --remote switch to 'git submodule update'.  This
allows you to always stay up to date (--remote) or to just use the
specific SHA-1 you last downloaded (no --remote).

So, we now surface this new feature in the UI.  It requires corresponding
changes to the git-client-plugin (Pull request 122) for this to compile.
Use SubmoduleUpdateCommand

The SubmoduleUpdateCommand pattern allows for easier understanding of what each
of the parameters means.
Add test for refs/head BranchSpec
Use JGit 3.3.1 and git-client 1.8.0
Fix javadoc warnings
Merge pull request #214 from dbroady1/trackingSubmodules

Add tracking submodules option
[maven-release-plugin] prepare release git-2.2.0
[maven-release-plugin] prepare for next development iteration