Num is now extends Expr

This way operators such as +, * and ** will work on it.
documented behaviour of Num to arithmetic

Which now works because it extends ``Expr``.
Started on symbolic differentiation.
support equality testing of Nums (and numbers)
lib for very basic simplification of expressions
simplify Num to the actual wrapped number
support symbolic subtraction
support simplification of sub and pow
proper simplification after differentiation
subtraction tests
bad exception name
more tests to increase coverage
updated README
fixed typo
more coverage on simplification
more simplification coverage
Merge branch 'develop'