#10 Additional flag for tasks and logged time / activities

[Stefan]

When logging a time / activity, I would like to be able to decide whether the activitiy will be billed / factored or not.
Each logged time will get a flag for factoring [yes | no]
For each defined task a default value for factoring will be set when adding the task in adminstration mode, whether this task normally will be charged or not. When logging a time for that task, I can choose whether to factore or not. I can overwrite the default value.
For the overview and export views, there will be an additional filter value for "factoring"