Parameter for coloring dendrogram above threshold

In the current code one can provide a color palette to be used for the clusters in `scipy.cluster.hierarchy.dendrogram` using the `set_link_color_palette` function in the same module. However, the parent tree, above the color threshold for clusters, is hard coded to the matplotlib color string `'b'.

This patch adds a parameter to `dendrogram` called `abv_threshold_color`, default still set to `'b'`, which changes this color.

As an example, I've attached an image where the parameter is set to `'lightgrey'`.
![figure_1](https://cloud.githubusercontent.com/assets/668803/3421603/a36b824e-fef5-11e3-8ad4-9d24aecff901.png)