Add basic math functions, Installation & Update instructions

Math functions accept some basic operands. Syntax:

```javascript
v,100*2 // Vertical guide at 200px
v,100/2 // Vertical guide at 50px
h,100-2 // Horizontal guide at 98px
h,100+2 // Horizontal guide at 102px
```

Partially resolves #2.

Also added some installation and update instructions.