Add shape param for LogN distribution

[mtrbean]

It is currently not possible to set the "usual" loc and scale params on the log scale for the LogNormal distribution.

loc is usually 0, and scale should usually set to exp(mu)

For details, see http://nbviewer.ipython.org/url/xweb.geos.ed.ac.uk/~jsteven5/blog/lognormal_distributions.ipynb

[tisimst]

While this is true for the built-in "LogN" function, you can input your own distribution directly like

a = rv(scipy.stats.lognorm(shape, loc=0, scale=scale))

I'll have to look into this more to justify the correction to the convenience function.