Compare to None by identity instead of equality

FutureWarning: comparison to `None` will result in an elementwise object
comparison in the future.
Don't ignore rcParams defaults in legend proxies
Return artists from all 1D plotting functions
Vastly simplify view limit settings

matplotlib's built-in system for inferring appropriate view limits from the
data and axis scales is quite powerful. Use it instead of setting the axis
limits by hand. This has the added advantage that subsequent calls to histogram
plotting functions can expand the view if necessary.
Merge pull request #3 from jvansanten/master

Fix minor plotting annoyances