Prefer Decimal over float in utils

[mhluongo]

In financial applications, it's important to avoid floating-precision math. Instead, stick with integers (and either don't divide, or round conscientiously) or use a good arbitrary-precision math library.

In Python, that library is decimal. It supports all the operators you'd expect, properly "coerces" integers into Decimals, and throws an exception if fixed-precision and floating-point math mix.

I didn't go through the whole code base- I just fixed a couple things in utils I wanted to use. The tests appear to be unaffected, though I'm not sure they have the coverage for a change like this, so use caution when merging.

[mflaxman]

Thanks @mhluongo and good call, this is better. Would you mind adding test coverage for these changes? I'm requiring test coverage of all new PRs.

I realize it's a bit unfair that you're improving something with no test coverage and it requires test coverage in order to be merged. If it's a pain, I'm happy to write the tests for you.

Thanks again @mhluongo!

[mhluongo]

I'll see if I have some time in the next couple days.

[mflaxman]

Thanks @mhluongo !

[eric20141111]

still not merged yet ......

[quentinlesceller]

I think that would still require some test coverage to be sure this is not breaking anything.