HEAD independent version script

[yuyichao]

A slightly better way to fix #151.

Do not rely on the current checkout version and consistently query version info for the commit specified (default to HEAD). The only requirement now is a clone of the julia repo with up-to-date commit and tag info (i.e. it needs to have the commit you specify and the latest tag before the commit).

Before,

yyc2:~/projects/julia/master
yuyichao% ~/projects/contrib/Compat.jl/bin/version.sh      
0.5.0-dev+0
yyc2:~/projects/julia/master
yuyichao% ~/projects/contrib/Compat.jl/bin/version.sh release-0.4  
0.5.0-dev+572
yyc2:~/projects/julia/master
yuyichao% ~/projects/contrib/Compat.jl/bin/version.sh HEAD~      
0.5.0-dev+2980
yyc2:~/projects/julia/master
yuyichao% ~/projects/contrib/Compat.jl/bin/version.sh 86cd5ea3c61426ebddd1d3809ffa869d6726fb30
0.5.0-dev+2048

After,

yyc2:~/projects/julia/master
yuyichao% ~/projects/contrib/Compat.jl/bin/version.sh
0.5.0-dev+2983
yyc2:~/projects/julia/master
yuyichao% ~/projects/contrib/Compat.jl/bin/version.sh release-0.4
0.4.4-pre+35
yyc2:~/projects/julia/master
yuyichao% ~/projects/contrib/Compat.jl/bin/version.sh HEAD~        
0.5.0-dev+2980
yyc2:~/projects/julia/master
yuyichao% ~/projects/contrib/Compat.jl/bin/version.sh 86cd5ea3c61426ebddd1d3809ffa869d6726fb30
0.5.0-dev+2048

[yuyichao]

Also update the README about the version script...