[Ml-yokadi] task recurrence

[Sébastien Renard]

Hello,

Please find on "recurrence" git branch a first implementation of recurring 
tasks.

New function : t_recurs. 
Ex. :
- t_recurs 452 daily 11:30
- t_recurs 452 weekly SA 11:30

Modified function : t_mark_done on a recurring task scheduled it for its next 
date.

Limitations/things to do : 
- monthly tasks not (yet) implemented. Quite easy to do (5 lines of code)
- complexe recurrence not available. Backend can handle it but it is quite 
tricky to find an appropriate syntax for t_reccurs.
- db migration script is not done (yet) but should be easy (add Recurrence 
table and foreign key.).
- recurrence table should fill up and a cleanup process should be defined
- some code and constant should be moved to dateutils

Comments welcomed.
-- 
Sébastien