Add a power function to math.js #122

Add power(base, exponent) to math.js beside the other functions, for whole-number exponents. A negative exponent should throw a RangeError.

Definition of done

• power(2, 10) returns 1024 and power(5, 0) returns 1
• power(2, -1) throws a RangeError
• Tests for each case in test.js