AOOSModel.php: Documentation + cache modes

AOOSModel.php: Documentation + cache modes

Added documentation for all data functions.
Added cache modes, which defines how calls to populate are handled. At the moment two cache modes are implemented:
static and dynamic. Cache modes are set using setCacheMode($cacheMode), which takes an integer parameter, which are
define()'ed. AOOSMODEL_CACHE_STATIC set the static cache mode and AOOSMODEL_CACHE_DYNAMIC sets the dynamic cache mode.
If cache mode is set to static, model is only populated when the model is empty. If cache mode is set to dynamic, model
is emptied and then populated on each call to populate().