Double cropping speed 

[As I suspected](https://github.com/minotar/imgd/pull/95#issuecomment-65904435), the (new?) built-in cropping method is much more efficient than the custom implemented one, and works just as well. It's also twice as fast.

Again, modified GetHead to run the crop 50000 time and Apache Benched 50 requests in series.
```
Before change: 567.038 [ms] (mean)
After change: 224.005 [ms] (mean)
```

Cheers :beers: