subtle change in command docstrings to fix hit expire
subtle change in command docstrings to fix hit expire

docopt wasn't parsing command as expected, causing all "hit extend"
commands to extend assignments, not time. Added parentheses around
pairs of optional parameters that have to appear together to fix the
problem.

Fixed similar problem in worker list, though don't think earlier version
caused problems in this case.

kachergis/psiTurk,andyh616/psiTurk,pfeyz/psiTurk,kachergis/psiTurk,johnmcdonnell/psiTurk,pfeyz/psiTurk,NYUCCL/psiTurk,NYUCCL/psiTurk,kachergis/psiTurk,michaelpacer/psiTurk,andyh616/psiTurk,suchow/psiTurk,mshvartsman/psiTurk,rbtrace/psiTurk,michaelpacer/psiTurk,johnmcdonnell/psiTurk,andyh616/psiTurk,michaelpacer/psiTurk,johnmcdonnell/psiTurk,rbtrace/psiTurk,NYUCCL/psiTurk,berkeley-cocosci/psiTurk,berkeley-cocosci/psiTurk,mshvartsman/psiTurk,pfeyz/psiTurk,mshvartsman/psiTurk,berkeley-cocosci/psiTurk,suchow/psiTurk,rbtrace/psiTurk,suchow/psiTurk