Changing all math to use BigDecimal for precision

Hey there, thanks for Kalc :)

I've been using it in a graphing project (https://ensight.exec.io) and we were getting floating point errors.  I forked your project and wrote a test for the error I was getting (line 77 of `spec/interpreter_spec.rb`)

The solution was migrating to Ruby BigDecimal class.  It handles everything you do in Floating Points but it gives you arbitrary precision decimal maths accuracy.

Hope you merge it :)