Use Cargo to run the benchmarks

Naming the benchmark folder benches instead of bench lets Cargo run the
benchmarks one by one like the test suite. The directory structure was modified
accordingly also getting rid of the now superfluous files.
Adapt Travis CI integration to benchmark change
Merge branch 'cargo_for_benchmarks' of git://github.com/aepsil0n/cgmath-rs
Improve the benchmark suite.

As a side effect, this adds `Rand` deriving to the benchmarked strucures.