Fix colorbar positioning when using 'axes' argument

Fixes #299. The fourth argument of add_axes was using y1 instead of a width (credits go to @AlexandreAbraham for finding out the root cause of this).

I tweak the margin logic to make it scale invariant. Updated plots:

![plot_stat_map_with_multiple_axes](https://cloud.githubusercontent.com/assets/1680079/5262417/d9aeac72-7a25-11e4-895e-dc91f56b00e2.png)

![plot_stat_map_default](https://cloud.githubusercontent.com/assets/1680079/5262419/de330b30-7a25-11e4-92fe-d106f6d3ebf5.png)