Add Pkg.whorequires to list packages depending on a particular package

Do not merge until the 0.4 window opens.

Demo:
```
julia> Pkg.whorequires("Color")
8-element Array{String,1}:
 "Cairo"      
 "Compose"    
 "Gadfly"     
 "Images"     
 "ImageView"  
 "ProfileView"
 "PyPlot"     
 "Winston"    
```

Now I know which packages should be tested if I'm contemplating a breaking change to Color.

CC @StefanKarpinski.