WIP: Oo interactive; enable interactive mode with Axes and Figure methods

[efiring]

This is a sketch illustrating one method of allowing one to use the OO interface in interactive mode. It involves adding three methods to the Figure and Axes classes, and then using a decorator for any method in those classes that should have a draw_if_interactive behavior. Typically this would include all such methods that are wrapped in pyplot. In the initial version of this PR, this is illustrated with Figure.suptitle, Figure.text, Axes.plot, and Axes.errorbar. Print statements are presently left in to show how the system works; in particular, note that even though errorbar calls plot, the drawing operation is executed only once, so there is no significant loss of efficiency compared to the present pyplot function. A simple illustration in ipython is:

%matplotlib
import matplotlib.pyplot as plt
fig = plt.gcf()
fig.text(0.5, 0.5, 'testing')
ax = plt.gca()
ax.plot([1,2,3])
ax.errorbar([1,2.5, 3.5], [2, 3.5, 2], yerr=2)

[tacaswell]

closing in favor of #4091