Allow passing an optional parent for inheritence

This allows for a scenario where you can create a base Entity in your project and have all modules inherit from that:

```Javascript
var Entity = require('mesia').persistence.Entity;

var MyClass = function() {
  // ...
}

Entity.inherit(MyClass);

var MyOtherClass = function() {
  // ...
}

Entity.inherit(MyClass, MyOtherClass);
```