ENH: Make plot_displacements more flexible; change default arrow appearance.

This reworks `plot_displacements`, a barely-documented function that makes plots like this:

![image](https://cloud.githubusercontent.com/assets/2279598/3113584/5490eeb0-e6dc-11e3-9a2b-b9c92b2c4aa4.png)

The most important feature added is the optional `scale` argument, which makes it possible to study small displacements by uniformly enlarging the arrows to some reasonable size.

Also, I replace matplotlib's `arrow` axis method with the more general `annotate`. Strangely, `arrow` gives much less control over the look over the arrows.

Note to selves: The whole `plots` file needs some attention -- PEP8 and general housekeeping. I only lightly edited it when merging it in from my `mr` repository. After we merge Rebecca's #104 I will do some cleanup.