Added description about Max-Age field in Options (#148)

In this commit, the behavior of Max-Age field is described based on its
different values.
Fixes #131

sessions.go

@@ -24,8 +24,9 @@ const flashesKey = "_flash"
 type Options struct {
 	Path   string
 	Domain string
-	// MaxAge=0 means no 'Max-Age' attribute specified.
-	// MaxAge<0 means delete cookie now, equivalently 'Max-Age: 0'.
+	// MaxAge=0 means no Max-Age attribute specified and the cookie will be
+	// deleted after the browser session ends.
+	// MaxAge<0 means delete cookie immediately.
 	// MaxAge>0 means Max-Age attribute present and given in seconds.
 	MaxAge   int
 	Secure   bool