Improved: database provider is now singleton (issue #49)
Improved: logger is now singleton.
Improved: session manager is now singleton.
Fixed: model loaded event could be raised more than one time.
Removed: unused require string.
Added: Singleton pattern implementation and its tests.
Improved: session manager, database provider and logger now uses new singleton pattern implementation.
Fixed: test due new changes related with singleton.
Fixed: session manager test fault
Added: Collection manager which will provide with singleton collection objects by name.