Adding getNumberOfDaysSinceStart() methods to TimePeriodWithWeekEnd class

Change-Id: Ic79bc4d27da69ab382658d4f6a73c6c619a78d97
Merge commit 'refs/changes/56/1856/3' of ssh://gerrit.tuleap.net:29418/tuleap into tuleap-stable-master

* ssh://gerrit.tuleap.net:29418/tuleap:
  Adding getNumberOfDaysSinceStart() methods to TimePeriodWithWeekEnd class

Change-Id: I042e5babaf7bd21cbc32abf4d7937436545df74a
Tuleap 6.10.99.5

Change-Id: I1b6559b9ce711b60edbf06d477fed417b6781a32