Support --jobs in 'fetch' and 'recurse' commands. 

BUG=115840
TEST=

Review URL: http://codereview.chromium.org/9560010

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@124657 0039d316-1c4b-4281-b951-d872f2087c98