Added a feature for checking out all the dependencies based on a specific commit.

Added --commit option to the update command.

  $ jsworkbench update --commit=<commit>

This causes `git reset --hard <commit>` on the current branch and  `git reset --hard <most recent commit before the specified commit>` on each dependency.

~/.jsworkbenchrc is ignored for this particular command.
It can be ignored in the build command as well, by adding the following option: 
 $ jsworkbench build --ignore-local

While this command works for normal codebases, some issues are found in SWFCrew.
This is because the command does not work if the URLs and branch names in build.json have been changed and the user attempts to roll back across the change.