f64 type, addition works with doubles and ints together

[erkkikeranen]

Solves #29

• Double data type (f64) (double and not float because Clojure JVM uses Double by default)

Addition with promotion to double works:

user=> (+ 1 1)
2
user=> (+ 1 1.2)
2.2
user=> (+ 1.1 1)
2.1