Delete space between panels in theme_bw()

The current behavior of theme_bw() takes the same panel.margin=unit(0.25,"lines") as theme_grey(). This setting makes sense for theme_grey() since you need white space as a border to distinguish the different panels. However, theme_bw() uses a grey line for the panel border, yielding the white space un-necessary. In fact, as shown below on page 63 of Envisioning Information by Edward Tufte, the "grid boxes" or space between panels, are a bad design (top). A better design is grey boxes around each panel with no white space between them (bottom).

![thick-facets-are-bad](https://f.cloud.github.com/assets/932850/870117/ce885c36-f820-11e2-9852-7719686c54a1.png)

On the left: the current default output when using theme_bw() in ggplot2. On the right: the same plot with theme_bw() after applying my patch, which changes only one line of code.

![space-nospace](https://f.cloud.github.com/assets/932850/870110/7068ffac-f820-11e2-894c-e8440d87c151.png)