[TW-78] Flexible due dates: (e.g. sprints)

_Simon VC on 2011-06-20T07:45:33Z says:_

I want to be able to do this:

task date xmas due:2011-12-25
task add Buy Wife present due:xmas

or:

task date sprint2 due:2011-11-28 // Add a new date alias
task add Complete Some Task due:sprint2 // Add a task due by end of sprint2
task date sprint2 due:2011-12-28 // push back the date a month