Benchmarks across versions

It would be great to maintain benchmarks to compare pull requests to `develop`, and to measure the improvements brought by a new revision.

[BenchmarkTools.jl](https://github.com/JuliaCI/BenchmarkTools.jl) seems useful to benchmark solver building blocks.

I think we need a different tool to compare the performance of solvers across versions and across branches, e.g., using aggregate measures, performance profiles, etc.

@abelsiqueira