Added common code file

Included in `common.js`:
- `sewi.inherits` - a helper function for class inheritance.
- `sewi.constants` - a dictionary to store all constants, including "magic strings".

Note: to use `inherits`, sub-classes must still call the constructor of their superclass, e.g.:
```javascript
function Oval() {
    this.x = 10;
    this.y = 20;
}

function Circle() {
    Oval.call(this); // calls superclass constructor
    this.x = 20;
}

sewi.inherits(Circle, Oval);

var circle = new Circle();

circle.x == 20; // true
circle.y == 20; // true
circle instanceof Oval // true
circle instanceof Circle // true
```