RcParams instances for matplotlib.style.use

External libraries may want to prepare styles dynamically. For example, `brewer2mpl` provides various colormaps. While it could create it's own context manager for temporarily changing the `color_cycle`, I think it makes more sense if `matplotlib.style.use` can work directly with `RcParams` instances.

```
>>> with matplotlib.style.context(matplotlib.RcParams(**{'text.usetex':False})):
...     print matplotlib.rcParams['text.usetex']
...      
False
```
With this addition, the name this argument is a bit misleading (actually, it was a bit misleading before, too). Maybe something like `style_spec`, or something similar. (By "spec", I mean specifier.)

Also, the description is a bit difficult to read because of all the different options. Maybe something like:

```
    style_spec : str, dict, or list
        str: The name of a style or a path/URL to a style file. For a list of
            available style names, see `style.available`.
        dict: Dictionary with valid key/value pairs in `matplotlib.rcParams`.
        list: List of style specifiers (str or dict) applied from first to
            last in the list.

```