Add rake tasks for running the benchmarks

Hi,

I wrote this yesterday while I was toying around with the benchmarks. But I see now the tasks directory has gone.

Is this something you would want to have in, and if so, where best to put it?


```
rake bench            # Run all benchmarks
rake bench:hash       # Benchmark bench:hash
rake bench:hash:each  # Benchmark bench:hash:each
rake bench:hash:get   # Benchmark bench:hash:get
rake bench:hash:put   # Benchmark bench:hash:put
rake bench:list       # Benchmark bench:list
rake bench:list:at    # Benchmark bench:list:at
rake bench:list:cons  # Benchmark bench:list:cons
```