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

EDIT: name changed to "callers"

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.whocalled("cyclezip")
3-element Array{(Any,Any),1}:
 (392,LineInfo("circle","/home/tim/.julia/v0.3/Compose/src/form.jl",139,false))
 (214,LineInfo("text","/home/tim/.julia/v0.3/Compose/src/form.jl",249,false))  
 (3,LineInfo("rectangle","/home/tim/.julia/v0.3/Compose/src/form.jl",93,false))
```