Refactor

* Separate formulas in their respective [categories](http://office.microsoft.com/en-001/excel-help/excel-functions-by-category-HA102752955.aspx);
* Fix several bugs;
* Cover 100% of the code with tests;
* Remove the website from this repository, should be separate;
* Write tests with mocha, no generator (more flexible);
* Remove moment and underscore.string dependencies;
* Different error handling

We do not expect you to accept this PR but would like to offer it anyway. We did this major refactor so it would be easier to continue to contribute. Glad to answer any questions that arise. This is still a drop-in replacement for your module.