Add support for more operators

[cjbassi]

This PR is based on the Minor refactoring PR I also created, with the only new commit being the latest one. There's a lot of changes in the last commit but it's summarized in the commit message and I added a lot of comments in the file to. I've done quite a bit of testing and it seems to work for all the operations. Let me know what you think! I really like this project and the use of eval() but I wanted to make it more robust and I think it turned out well. Let me know if anything is confusing or if anything should be changed. I didn't change the README but that would have to be changed too.