Issue #38: Cluster the treks when zoom level < 12
Issue #38: custom icons
Issue #38 & #40: display markers according to the filtered treks
Issue #38: non clickable trek icon
re-activate the click event on trek's click
Merge branch 'master' into cluster_treks_low_zoom

Conflicts:
	app/scripts/map/controllers.js
Merge branch 'cluster_treks_low_zoom'