Added gc_stats basic functionality

I've added some stats, they show some important metrics on how efficient the GC is at freeing the data compared to how much data was used at the time, I believe this is where most optimization opportunities are.

From my tests at https://github.com/etcimon/druntime.gctest I've observed that the GC frees on average 3% of used memory per collection cycle. I've suggested sampling as a solution to reduce the amount of inefficient collections.
![collections](https://cloud.githubusercontent.com/assets/4785754/3070283/aa42cd9e-e2a7-11e3-9039-a506963ca27e.png)