Add method/macro for comparing two functions/expressions

Example use and output:

```
julia> e1 = quote
       for i in 1:100
          sum([1:10])
       end
       end
quote  # line 2:
    for i = 1:100 # line 3:
        sum([1:10])
    end
end

julia> e2 = quote
       for i in 1:10 
          sum([1:10])
       end
       end
quote  # line 2:
    for i = 1:10 # line 3:
        sum([1:10])
    end
end

julia> @benchmark(e1,e2,"compare","mil","cien",300)
2x12 DataFrame:
        BenchmarkCategory BenchmarkName Iterations  TotalWall AverageWall    MaxWall MinWall             Timestamp            JuliaVersion    JuliaHash CodeHash      OS
[1,]            "compare"         "mil"        283 1.40667e-5  4.97056e-8 1.19209e-6     0.0 "2013-01-06 02:46:02" "0.0.0+106299916.rdf56" "df564000a7"       NA "Linux"
[2,]            "compare"        "cien"        317 1.28746e-5  4.06139e-8 1.90735e-6     0.0 "2013-01-06 02:46:02" "0.0.0+106299916.rdf56" "df564000a7"       NA "Linux"

julia> function f()        
       sum([1:1000])
       end

julia> function g()        
       sum([1:100])
       end

julia> benchmark(f,g,"ej","mil","cien",300) 
2x12 DataFrame:
        BenchmarkCategory BenchmarkName Iterations   TotalWall AverageWall    MaxWall    MinWall             Timestamp            JuliaVersion    JuliaHash CodeHash      OS
[1,]                 "ej"         "mil"        318 0.000624418  1.96358e-6 1.09673e-5 9.53674e-7 "2013-01-06 02:46:58" "0.0.0+106299916.rdf56" "df564000a7"       NA "Linux"
[2,]                 "ej"        "cien"        282  7.65324e-5  2.71391e-7 1.19209e-6        0.0 "2013-01-06 02:46:58" "0.0.0+106299916.rdf56" "df564000a7"       NA "Linux"
```