Facet titles

This implements a `switch_strips` argument to `facet_wrap` and `facet_grid`, which can be set to `x`, `y` or `both` (the latter only for grids). This allows to display the facet strips near the axes. They then act as axes subtitles. The padding between the switched strips and their corresponding axes can be adjusted through a new theme setting.

Displaying the facet titles near the axes especially makes sense when the facet title directly characterize a change in the axis (for example, a transformation, a different measure, etc). This should be typically used in conjunction with a theme without boxes around the strips. I go over this in the updated documentation.

A grid facetting with strips switched to both axes:
![facet1](https://cloud.githubusercontent.com/assets/4465050/3280482/1c1e20c4-f466-11e3-8718-a9dcbd88b551.png)

A wrap facetting with free scales and strips switched to the x axis:
![facet2](https://cloud.githubusercontent.com/assets/4465050/3280481/1c1dcd0e-f466-11e3-9ae3-403ca7b3d9a3.png)