Add native Integer.toString(int) and Long.toString(long)

Hello Phil,

another try, please merge the following two native methods (similar to ```Double.toString(double)```)
- ```Integer.toString(int)``` and
- ```Long.toString(long)```

All test cases (DoubleToStringTest, FloatToStringTest, IntegerToStringTest and LongToStringTest) pass only if there are two or less kernels!
e.g., ```for(int i = 3; i < 5; ++i){```

```
List<Kernel> ret = new ArrayList<Kernel>();
for(int i = 0; i < 5; ++i){
  ret.add(new IntegerToStringRunOnGpu(i));
}
```
Maybe this issue belongs to the new multiple GPU runtime?

Thanks!

Martin