change all plt to pyplot

also renamed module 4 lessons to add prepended module number for
consistency across modules.  all notebooks have been run for
presentation
Merge pull request #116 from numerical-mooc/pyplot

change all plt to pyplot
Merge branch 'master' of https://github.com/numerical-mooc/numerical-mooc into numerical-mooc-master

Conflicts:
	lessons/02_spacetime/02_01_1DConvection.ipynb
Completed module 2