Refactored sync command to update command

Due to bug in git-subtree we cannot use 'git merge -s subtree' for updating module.
Instead we have to use 'git read-tree'. This patch removes 'sync' command and adds
'update' which is more stable and simpler.

For more info check:
http://stackoverflow.com/questions/1306595/git-confused-when-merging-an-update-into-my-subtree
http://stackoverflow.com/questions/15823730/git-subtree-merging-reports-conflict-when-merging-a-simple-upstream-change