#7 move "hourly rate" from project to task

[Olaf Willuhn]

At the moment you can set the hourly rate for projects and
users. If both is set, the project rate has a higher priority.

Wish: move "rate" attribute from project to task table.

Resolution: To preserve backward compatibility there should be
three possible rates: task, project, user (in this order).
Should be simple to implement - just add attribute "rate" to table
task and modify task::getRate()