Add an option to ah_helpers to make use of git at all (i.e. for updating submodules) optional
Add an option to ah_helpers to make use of git at all (i.e. for updating submodules) optional

larrybradley/astropy-helpers,larrybradley/astropy-helpers,embray/astropy_helpers,dpshelio/astropy-helpers,embray/astropy_helpers,larrybradley/astropy-helpers,bsipocz/astropy-helpers,embray/astropy_helpers,bsipocz/astropy-helpers,astropy/astropy-helpers,bsipocz/astropy-helpers,Cadair/astropy-helpers,Cadair/astropy-helpers,astropy/astropy-helpers,embray/astropy_helpers,dpshelio/astropy-helpers