change angle to gamma in force balance fig

there is still some confusion with angles in the text -- I updated the
figure showing forces to name the flight-path angle gamma (while
discussion above used theta for pitch angle, positive up!)
explain we follow Milne-Thompson/ centrifugal force

Derivation uses theta as positive upwards, and forces are in a rotating
frame, so we should properly speak of centrifugal force
checking a sign, executed style, no changes
Went through first two modules
Added gitignore for files inside lessons
removed pyc file
added corrections
Restored module 01_01
merge suggested changes to phugoid 02
merge changes to phugoid #2 suggested by @Yurlungur
fix fencepost error in phugoid nb2
Merge pull request #15 from numerical-mooc/phugoid.wip.gil

fix fencepost error in phugoid nb2
fixed integration loop so that it doesn't overwrite initial data
Merge pull request #16 from Yurlungur/master

fixed integration loop so that it doesn't overwrite initial data