new feature: named figures - #100

I've closed the previously opened issue (that didn't contain any code):

this small addition will make it possible to use explicitly named figures in pyplot.figure

Example:

plt.figure("today")
opens new or existing figure with label "today".
In windowed backends, the title of the figure window will be set to "today" instead of Figure 1.
Internally all figures remain tracked using fig.number.

The expected benefit of this patch is to make it easier to switch between multiple figures.

I've now added tests >> The code is ready for review.

Nice. I think plt.close('today') should be added, though, for completeness.

the commits seem to have messed up a bit !

This can't be merged in its current state. Please redo commit a2e82eb without adding the example files.