Dup plots patch

Patch to fix stevengj/PyPlot.jl#52. I'm not sure this is a very good and general way to do it, but I've at least found the problem, and perhaps this could be a platform for discussing a proper fix.

The issue is that with the hashing changes in Julia, the `redisplay` function no longer notices when a Figure type is already in the display queue. I suppose that properly hashing python objects is the real issue.

The fix here is to compare the output from the Python `__repr__` method, and fall back to the normal Julia equality test for types that aren't derived from Python.

Advice as to how to do this properly appreciated.