Update git submodules when updating plugins

[guillaumealgis]

As discussed in #454, plugins using git submodules need to run git submodule update after a fetch to update their dependencies to the right commit.

git submodule update
Update the registered submodules to match what the superproject expects by cloning missing submodules and updating the working tree of the submodules.

This is also related to #465, which fixed the initial clone for plugins using submodules.

Changes:

• Add option --recurse-submodules=on-demand to the fetch command to recursively fetch submodules commits;
• Add command git submodule update after successful git fetch and git reset --hard to make the submodules checkout the right commits.

Note: Not a big fan of the pyramid-callbacks-style in +[ATZGit updateLocalProject:revision:completion:], if you have a better idea, I'll take it :)