adding ability to choose your decimal places

When you do this adder: adder(0, 6.52, 6) it was returning this [0, 1, 3, 4, 5, 7].
But those values we not close enough to even intervals for what I needed so with this change you can do this: adder(0, 6.52, 6, 1) it will return this [ 0, 1.3, 2.6 , 3.9, 5.2, 6.5 ].

I have added tests to cover this case and verified that the original test cases maintain.

![](http://37.media.tumblr.com/31e177a9ac17e46c21364d200af6f980/tumblr_n30rc0Ig771qdlh1io1_400.gif)