RPN Expression Evaluator

Class for evaluating expressions given in string using RPN (Reverse Polish Notation).

TODO

1. DONE - add tests and example driver app
2. DONE - prune linear equation solver from solution
3. DONE - put in more test for verifying unary minus functionality
4. DONE - implement correct handling of unary minus
5. DONE - add larger set of functions
6. DONE - add support for "comma operator", enabling functions with multiple parameters
7. DONE - add few functions of that kind
8. DONE - add tests to verify multiparameter function functionality
9. DONE - make it a CMake project
10. DONE - clone it on Linux/Mac and work on it a bit