Intelligent slider size

[TomNicholas]

Attempts to fix #32 by automatically choosing the length of the slider based on the length of the slider label and value text.

There's a unit test which interrogates the figure and checks that the right side of the slider and text are not to the right hand side of the left side of the button.

The result should look something like this:

(This change seems to pass the image comparison tests on my machine even though the slider should be a slightly different size which is odd?)

[ianhi]

Have you considered a PR of this or something similar to core matplotlib? Being able to constrain a slider + text to be within a certain box is a generically useful feature.