Improve documentation for mplot, added example
Improve documentation for mplot, added example

also fixed type descriptors,
and changed fuction to return figure instance,
and made docstring conform to numpydoc standard

sjsrey/pysal,ljwolf/pysal,weikang9009/pysal,pysal/pysal,lixun910/pysal,ljwolf/pysal,ljwolf/pysal,lanselin/pysal,lixun910/pysal,lixun910/pysal,pedrovma/pysal