More operators and a Taylor expansion property

This week Yingfu and Adam continued working on the symbolic differentiation implementation in Scala.  The expression type was extended with exponentiation, the power operation and logarithms.  A memoized version of the evaluation function was also added.  More smart constructors were added along with a new differential function which computes higher order derivatives.  A new property-based test based on Taylor expansion was introduced.  It checks the difference between a function's value and the generated polynomial's value in a certain point.  This test was so far applied to unary functions and binary operators.