Display overtime as a colored diff on the booking page

We display the overtime as the difference between booked hours and
expected worked hours. The value is colored accordingly in red if the
user hasn't worked enough or blue is he has overtime.
Better wording for the overtime summary block on the front page
Disply current month name instead of "this month"
Request summary block infos only if dom is still there
Merge pull request #374 from dsp/summaryBlockAdjustments

Summary block adjustments