SQL and model optimisation

Big effect (on speed and memory), but few changes :-)
Drastically reduced the number of SQL requests needed (from 233 down to
8 to load the home page with my own data set = 140 feeds in 15
categories).
Drastically reduced the amount of data transferred from MySQL to PHP.
Disable SQL LIMIT optimisation during search

This patch is to make search work again after the new SQL optimisations,
by removing some of the optimisations when searching is used.
Optimisation of search is left for some future work.
The whole base is indeed transfered from MySQL to PHP, which is not
good.
Work around

In the current SQL request with LIMIT, if many dates are identical, the
pagination may not work properly. Added a little more tolerance, but
will have to be solved better.
Handle paging for entries with identical date

Paging now works even when many entries have the same date.
SQL speed could probably be improved by testing first on date, and then
on CONCAT.
Also, having an index on date would probably help too.
Correct bug: last category was not shown
Corrected bug with the default "No Category"

The SQL optimisation patch had introduced a bug with "No Category", now
solved
Merge branch 'SqlOptimisation' of https://github.com/Alkarex/FreshRSS into Alkarex-SqlOptimisation

Conflicts:
	app/controllers/feedController.php
Issue #155 : suppression fonctions commentées
Issue #155 : correction fonction _nbNotRead

intval renvoyant toujours un integer, le test is_int() passait toujours,
c'est corrigé maintenant
Merge branch 'Alkarex-SqlOptimisation' into sql-optimisation
Issue #155 : correction bug prefix table SQL

Le préfixe freshrss_ était rentré en dur dans une des requêtes
Merge branch 'sql-optimisation' into dev