new funcs, error messages and tests

[mpousmali]

Hi there! I added exp(x), factorial, mod operation, pi, decimal point and backspace. Also, I added tests for the new functions and I fixed some issues of the already existing tests. Moreover, I added error messages in calculator if someone try to calculate an undefined expression (for example factorial of negative number or tan(90)). I have enclosed a screenshot of the calculator after the changes. It would be my pleasure, if you accept my pull request. Feel free to indicate new issues or recommend new ideas!