Different DB backends are used by CW, mainly postgresql and sqlite.
According to RQL's documentation:

    Any N WHERE N is Note, N written_on D, D day> (today -10), N written_by P, P name 'jphc' or P name 'ocy'

should return all the notes less than 10 days old written by jphc or ocy.

but, as described in #109, it only works with postgresql, which is *not*
mentioned in the documentation.

TODAY is working well in sqlite, but not the operations with TODAY.
TODAY is translated to 'CURRENT_DATE' in sqlite, and `SELECT CURRENT_DATE - 3`
return the current year minus three, and *not the current date minus 3 days* as
expected according to the documentation.

A lot of tests in CW are performed with SQLite (because it's fast to set up!),
so… let's make it work !

This patch suggests to adapt the 'visit_mathexpression' method, to write the
expected SQL when were are dealing with:

- sqlite
- a date(time) on the left (left-hand-side, lhs)
- a integer (treated as an amount of days) on the right (right-hand-side, rhs)


The tests have been updated to check that the conversion rql to sql is the
expected one, according to the backend.


close #109