New autoscaling and offset capabilities for collections

Potential testers:

CVS mpl now has a new demo (examples/ that illustrates two minor new capabilities:

1) The axes.add_collection() method accepts a new boolean kwarg, "autolim"; if True, then a subsequent call to axes.autoscale_view will take into account the data in the collection.

2) LineCollection can now be called with kwargs
      "offsets=(xo,yo), transOffset=None"
to facilitate making waterfall plots or stagger plots--that is, a set of sequentially offset curves.

The demo may also be useful in illustrating some basic ways of using line and polygon collections; this is not entirely obvious from the docstrings. (I have tried to improve some of the docstrings also.)

The changes were prompted by Chris Barker's earlier message with a demo script, which showed me how to use the two transforms involved in these types of collections.