(Fix #33, #35) A more flexible definition of isvector(v)

Now returns true if `v' is a table and both `v.x' and `v.y' are numbers.

vector.lua

@@ -36,7 +36,7 @@ end
 local zero = new(0,0)
 
 local function isvector(v)
-	return getmetatable(v) == vector
+	return type(v) == 'table' and type(v.x) == 'number' and type(v.y) == 'number'
 end
 
 function vector:clone()