ENH: Context manager for styles.

This is a small PR to wrap `style.use` in a context manager. With it, you can change plotting styles locally by 
```python
with style.Style('ggplot'):
    fig = figure()
    ax = fig.add_subplot(111)
    ax.plot(x, y)
```
or
```python
s = style.Style('jfm')
s.activate()
plot(...)
s.deactivate()
```