Rename log_warning to log_warn
Change error to warning
Fix typos in declarations (issue #41)

GetServerVarAsbool -> GetServerVarAsBool
WEAPON_FIREEXTINGUISHR -> WEAPON_FIREEXTINGUISHER
Add the IsValidVehicle() native (issue #41)

It's not declared in SA-MP include but exists, according to this Wiki page:

http://wiki.sa-mp.com/wiki/IsValidVehicle