Reduce time scaling rate for batch crafting

I was messing around with drbig's batch crafting time reduction changes, and noticed that ~85% of the time reduction happened within the first 2-3 items of the batch.

By tweaking the exponential curve of the `batch_time` method, I was able to smooth out the rate the the time reduction approaches the `batch_rsize` limit, and still have it make sense.

The `batch_rsize` is now the point where 99.5% of the `batch_rscale` time reduction factor is applied:

For reference you can see a graph of the old curve:
http://www.wolframalpha.com/input/?i=y+%3D+2.0+%2F+%281.0+%2B+e^%28-12x%29%29+-+1%2C+x+in+{0%2C1}

vs the new curve:
http://www.wolframalpha.com/input/?i=y+%3D+2.0+%2F+%281.0+%2B+e^%28-6x%29%29+-+1%2C+x+in+{0%2C1}