Add more benchmarks.

Note that this PR includes #135.

I wanted to compare the performances of `cgmath` vs. `nalgebra`, so I wrote benchmarks for some of their common features. Instead of using hard-coded inputs (like the old benchmarks), I generate 8,192 of them randomly and use one of them at each iteration of the `Bencher`. As a side effect, this makes vectors, matrices, and quaternion, derive from `Rand`. I am not sure you wanted this.

Fix #134.