clustermap: side colors should ignore custom centering

Currently the `center` keyword is passed to the side color heatmaps. This can cause some unexpected formatting of the side colors. Since `center` is defining a custom colormap and should only affect the heatmap, best to pop it off.

For example. using the `flight_rect` example from the documentation:

```
sns.clustermap(flights_rect, col_colors=col_colors, row_colors=row_colors)
```
![fig1](https://cloud.githubusercontent.com/assets/755664/5189369/49033082-74ab-11e4-8314-3db6d837d91f.png)

```
sns.clustermap(flights_rect, center=flights_rect.loc["January", 1955],
                    col_colors=col_colors, row_colors=row_colors)
```
![fig2](https://cloud.githubusercontent.com/assets/755664/5189372/4d91403a-74ab-11e4-93a4-e334d2d7cfc0.png)