BUG: Add subplot spec  eq  

See:  http://stackoverflow.com/questions/20017314/how-are-existing-subplot-axes-retrieved-using-subplot2grid-gridspec-in-matplotli

in short, 

```
gs = gridspec.GridSpec(2,1)
In [220]: gs[0, 0] == gs[0, 0]
Out[220]: False
```

which breaks the logic of calling `ax = fig.add_subplot(gs[0,0])` multiple times.