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 remote-tracking branch 'upstream/master'

Conflicts:
	lessons/01_phugoid/01_02_Phugoid_Oscillation.ipynb
	lessons/01_phugoid/01_04_Second_Order_Methods.ipynb
	lessons/02_spacetime/02_01_1DConvection.ipynb
	lessons/02_spacetime/02_02_CFLCondition.ipynb
	lessons/02_spacetime/02_03_1DDiffusion.ipynb
	lessons/02_spacetime/02_04_1DBurgers.ipynb