rewrite the hook in pure sh shell

the pure shell makes it a bit more readable,
but I do not expect you to merge it - it's just mine opinion
move the loading error to ruby to avoid the double ruby initialization penalty
fix the hook test
Merge branch 'master' into patch-1
allow overriding the used ruby command in the hook, fix #97
add flag to silence the rvm path warning, fix wayneeseguin/rvm#2428
the warning is triggered by the PATH modification by git to be the first one,
adding the flag makes no effect on any other software and fixes the warning
for rvm usesrs.
Merge pull request #98 from mpapis/patch-1

rewrite the hook in pure sh shell