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.
Merge pull request #16 from Alkarex/SqlOptimisation

SQL and model optimisation