Allow grouping of benches

[devinus]

E.g.

group "Strings" do
  bench "Poison" do
    # ...
  end
end

EncoderBench.Strings.Poison:              5000   384.85 µs/op

[alco]

The problem with grouping is that there are different use cases. Sometimes you want to group different implementations of the same algorithm. Other times, you want to group string-related functions in one place.

For now you'll have to use modules, I'm also planning to add tags of some sort. But the most flexible grouping will most likely be implemented on the HTML page generated by bench.graph.