Add Profile.whocalls to determine main callers of a particular function

This is somewhat like the Profiling analog of `@which`. It's quite useful in aggregating results from a complex sequence of runs.

Demo: in `Gadfly/test`,
```
julia> @profile include("test.jl")

julia> Profile.whocalls("cyclezip")
3-element Array{(Any,Any),1}:
 (LineInfo("circle","/home/tim/.julia/v0.3/Compose/src/form.jl",139,false),392)
 (LineInfo("text","/home/tim/.julia/v0.3/Compose/src/form.jl",249,false),214)  
 (LineInfo("rectangle","/home/tim/.julia/v0.3/Compose/src/form.jl",93,false),3)
```