allow packages to be added from repos and using git revs, for example: (#175)
pkg> add Example#master
pkg> add Example#43oi54h3254
pkg> add https://github.com/fredrikekre/ImportMacros.jl#branch
Packages tracking branches will be updated on major upgrades
(cherry picked from commit b20636de040d321d1b935177d6dc8e2513644f38)