Add now for current datetime

src/date.rs

@@ -200,3 +200,7 @@ pub fn to_duration(num: &Number) -> Result<Duration, String> {
     let num: Option<i64> = (&(&num.0.get_num() / &num.0.get_den())).into();
     Ok(Duration::seconds(num.unwrap()))
 }
+
+pub fn now() -> DateTime<FixedOffset> {
+    UTC::now().with_timezone(&FixedOffset::east(0))
+}

src/eval.rs

@@ -280,6 +280,7 @@ impl Context {
         }
 
         match *expr {
+            Expr::Unit(ref name) if name == "now" => Ok(Value::DateTime(date::now())),
             Expr::Unit(ref name) => self.lookup(name).ok_or(format!("Unknown unit {}", name)).map(Value::Number),
             Expr::Quote(ref name) => Ok(Value::Number(Number::one_unit(Rc::new(name.clone())))),
             Expr::Const(ref num, ref frac, ref exp) =>