Add mathematical functions to the index page.

Reference: [Trello](https://trello.com/card/missing-items-on-command-und-function-index/4ff142712c442e040466599d/13)

Added functions:

* Abs()
* Ceil()
* Exp()
* Floor()
* Log()
* Ln()
* Mod()
* Round()
* Sqrt()
* Sin()
* Cos()
* Tan()
* ASin()
* ACos()
* ATan()