Add support for submodules

In our company we use a lot of solutions of yours: jira, confluence, crowd, fisheye and now we are taking look at the source tree: our first look was great, but one small thing blocks us from using it at the end: no support for submodules.

In our projects we have currently 5-6 submodules linked. For us it's important to know which of them have the right version checked-out as for the commit in the umbrella project. So beside displaying all submodules, their status, some other minor stuff would be appreciated: update submodule, update all of them, initialize them. Having this feature, I'll make sure the whole team of ours (20 ppl) will use source tree instead of GitX (modified especially by us by adding submodules support) or Tower.