First visualization widgets

This PR adds a first set of IPython widgets for visualization.

- `browse_images`:  allows us to visually navigate through a list of images.
- ` visualize_aam`: displays and `AAM`.
- `visualize_fitted_images`: allows to visually asses the result of a particular fitting procedure.
- `visualize_iter_images`: allows to navigate through the partial fitted shapes obtained at each iteration of a particular fitting procedure.
-  `plot_ced`: plot the Cumulative Error Distribution (CED) graph.

Some of these widgets implement part of the functionality lost by `(Multi) FittingResults` in #378. There exist some coupling between these widgets and `Matplotlib` that might need to be discussed (specially for `plot_ced`) 

> This PR is placed on top of #411 and #378