grid used w/ date_axis not working well

grid not lining up with the ticks
also, ticks not showing up on date_axis

```r
x <- seq(as.Date("2013-01-01"), as.Date("2013-10-02"), "days")
y <- rnorm(length(x), 71, 19)
gs <- gsplot() %>%
    points(x, y) %>%
    date_axis(side=1, pos.lab="interval", tick.int="month", snap.to="month") %>%
    axis(side=c(3,4), labels = FALSE) %>%
    grid()
gs
```