Added benchmarks

So a benchmark suite would be pretty handy so performance can be reproducibly measured. 

    [info] 50% Scenario{vm=java, trial=0, benchmark=UnsafeValueAt} 7460.54 ns; σ=74.13 ns @ 3 trials
    [info] 75% Scenario{vm=java, trial=0, benchmark=Apply} 8312.16 ns; σ=93.43 ns @ 10 trials

This pull request adds one. Currently only has a few simple benchmarks for `DenseVector`, but as performance improvements are made more benchmarks can be added. 

Since there is no good, standard way of benchmarking in Scala, I've stolen the benchmark structure from Spire. There is a benchmark subproject and benchmarks are run via:

    > project benchmark
    > run

It was necessary to restructure the project, having a `breeze` and `benchmark` subproject. As I said, I just stole that from spire, not attached to that structure.