Add benchmarks when using mixology vs. extend vs. include.

[JoelJuliano]

The benchmarks are based on https://github.com/mikepack/dci_vs_include, it compares extend vs. include intended measurements for using DCI.

I added the mixology gem to compare the results when using mixin vs. extend.

Here's my results

ruby ./dci_bm.rb user system total real DCI 16.580000 0.100000 16.680000 ( 16.919118) DCI 16.500000 0.100000 16.600000 ( 16.817986) DCI 16.490000 0.100000 16.590000 ( 16.841119)

ruby ./dci_bm_mixology.rb user system total real DCI 16.350000 0.140000 16.490000 ( 16.785906) DCI 16.300000 0.130000 16.430000 ( 16.752707) DCI 16.310000 0.140000 16.450000 ( 16.807520)