[Ml-yokadi] New recurrence storage layer

[Aurélien Gâteau]

A few more details on this dev-only update:

Sébastien Renard wrote:
> (1) before pulling code (or go backward to b809b3c97e1dc89) note all the task 
> that have recurrence. You can get id with this sql request: select t.id from 
> recurrence r, task t where r.id=t.recurrence_id;

It's important to do this *before* the checkout if you want to print the
task details within Yokadi so that it's easier to recreate the recurrences.
> 
> (2) with sqlite3, connect and drop recurrence table

This means:

  sqlite3 /path/to/yokadi.db
  drop table recurrence;

(2.5) missing command to reset the recurrence_id of the task table:

  update task set recurrence_id = null;

> 
> (3) checkout master.
>


> (4) open python shell, cd to src/yokadi and import db
> 
> (5) open your database with db.connectDatabase(<path to .db>) and create the 
> new table with : db.Recurrence.createTable()

More detailed version of step (4) and (5):
1. cd to src/yokadi
2. start python
3. type this:

  import db
  db.connectDatabase("/path/to/yokadi.db")
  db.Recurrence.createTable()

> 
> (6) retype your recurrence ;-)
> 
> a+