Set up work for the gravity effector.

Task @T22 : Set up the files for the gravity effector. No real
"work" done yet.
Created the norm_gravity effector class.

Task @T22 : Completed the nrtb::norm_gravity struct, an effector
which applies a -1 g acceleration along the z axis to simulate
earth normal gravity.
Completed the gravity module's unit test.

Task @T23:R : The gravity effector is now completed, unit tested and
ready to go.
Added gravity_effector to the sim_engine Makefile.

Task @T24 : This completes the changes implementing the gravity
effector. Gravity effector is now unit tested and ready to merge.
Merge pull request #10 from fpstovall/gravity_effector

Gravity effector completed and ready to go.