Added Address.postalCode

A quick and dirty generator of UK postal codes.
The format defaults to LLD DDL.
L's get replaced with a letter and D's get replaced with a digit.

Examples:

```
var Faker = require('Faker');
Faker.Address.postalCode(); // VF2 2BY
Faker.Address.postalCode('LLDD DLL'); // FQ76 5ST
```