Allow for null-strides in wireframe plot

When making a wireframe plot, we've found that it's often nice to have just rowstrides or just column strides.  Currently, a value of 0 for the stride distance will result in an error.

This small change seems all that is needed.  Is there a reason for not wanting this to be allowed?  

One issue I foresee is that if users pass rstride=None, cstride=None, they will get blank plot.  I hope it would be obvious to them as to what is occurring in this case.  

I've attached  a screenshot of a wireframe plot with no column stride.
![no_stride_plot](https://cloud.githubusercontent.com/assets/1972276/4363657/0d38f45a-4298-11e4-9854-fc16cb91e6fa.png)

This variable is undefined at this point, isn't it?