Text progressbar library for python.

This library provides a text mode progressbar. This is tipically used
to display the progress of a long running operation, providing a
visual clue that processing is underway.

The ProgressBar class manages the progress, and the format of the line
is given by a number of widgets. A widget is an object that may
display diferently depending on the state of the progress. There are
three types of widget:
- a string, which always shows itself;
- a ProgressBarWidget, which may return a diferent value every time
it's update method is called; and
- a ProgressBarWidgetHFill, which is like ProgressBarWidget, except it
expands to fill the remaining width of the line.

The progressbar module is very easy to use, yet very powerful. And
automatically supports features like auto-resizing when available.
