Add theme to ggpairs

add themes to ggpairs objects

```{r}
data(tips, package = "reshape")
pm <- ggpairs(tips[,2:3])
pm # original
```
![screenshot 2014-11-20 15 51 51](https://cloud.githubusercontent.com/assets/93231/5132744/7db8b7fc-70cd-11e4-9443-2f253768e30c.png)


```{r}
## change to black and white theme
pm + ggplot2::theme_bw()
```
![screenshot 2014-11-20 15 52 06](https://cloud.githubusercontent.com/assets/93231/5132747/8150dc6e-70cd-11e4-95af-8ca02702e2da.png)


```{r}
## change to linedraw theme
pm + ggplot2::theme_linedraw()
```
![screenshot 2014-11-20 15 52 21](https://cloud.githubusercontent.com/assets/93231/5132750/85a87092-70cd-11e4-9045-9eb2ec81f302.png)


```{r}
## change to custom theme
pm + ggplot2::theme(panel.background = ggplot2::element_rect(fill = "lightblue"))
```
![screenshot 2014-11-20 15 52 34](https://cloud.githubusercontent.com/assets/93231/5132752/899a69b2-70cd-11e4-8153-ed79496a1759.png)